How do I handle this instruction

New policies are invented for a reason: to reduce costs, improve quality, increase customer satisfaction, deliver faster, retain good employees, etc etc. You don't make it clear in your question what your issue is with this policy. It will fail to achieve those aims? It will introduce negatives that outweigh those aims? The company doesn't need to achieve those aims? There are different policies that would achieve those aims with less negatives?

It's important that you understand the purpose of the policy (eg to reduce costs) and exactly why you oppose it (we don't need to reduce costs, it won't reduce costs, there are better ways to reduce costs) before speaking to your people.

Once you understand that, your instructions are, sticking with the example of reducing costs, to tell people "I want you to follow this new policy, which is being introduced to reduce costs" and not to include "even though I don't think we need to reduce them at all" or "even though I think it will fail to do so" or "even though many better options exist to reduce costs." Just give the high-level reasoning behind the policy. Passive tense ("is being introduced to") will do a lot of work for you.

Saying things like "I know this is foolish but management insists on it" will help no-one. You don't need to provide a zillion details of why this is the best policy ever. You also don't need to say you fought it, you tried, "sorry team I didn't want this but we're stuck with it" or the like. While it feels like lying to omit these sentences, they make things worse. If you truly feel "Fair enough, their prerogative" then you have to walk that walk. Folks with more experience and possibly more information than you believe that policy A will achieve aim B, and that aim B is worth achieving. That is the message you are being asked to deliver.

When people tell you "but it won't reduce costs!" or "but our costs don't need to be reduced!" or "wouldn't it be better to do this other thing instead?" you don't need to either agree or disagree with them. You can say something as simple as "this is the new policy and I am asking you to follow it."

“I have been told to tell you that I want you to adhere to this policy, and justify to you why this policy is right. I have not been told in any way by upper management to tell you this, and I will not tell you in any way to believe that upper management has anything to do with it”.

In other words, lie to your staff in the most transparently obvious way possible.

I feel you. More precisely I feel like you are my boss. Because I just read the announcement of how we have a new Return-To-Office policy from our coprporate overlords. And how this is a great chance for us to finally... grow together as a team even more (official fairy tale) or more realisticly hate each other because we will now be penned up in bunches of 4-6 in a room with no air condition and constant phone calls that we took about an hour of traffic to even reach. Compared to the previous years of single-person air conditioned offices with no distractions at our respective homes with zero traffic.

I don't think there is a good way to transport this decision, simply because it is not logical or in the best interest of the product the company is making.

Write an official communication (E-Mail, MS Teams, whatever your official channel is) doing exactly what your bosses want. Give it your best sugar coating. Bring all their arguments and "explain" how they make total sense. It doesn't matter that it is total bullshit. You don't believe it, your coworkers don't believe it, it doesn't matter. These were your orders, you follow them.

Before you post that, call in a team meeting with no notes and no recording and tell your team that you think it sucks and you fought it, but weren't successful. Your job now is to make it work and take a boatload of lipstick to the pig. Tell them you will post that after the meeting. And if they want to bitch and moan about it, they will need to do it the old fashioned way, with a coffee in the smokers corner outside the office building. You will be there to listen and maybe chime in.

Then post it.

Whether you want to look for a better job is up to you. I'm not entirely sure myself yet. Do not make the mistake of actually trying to do what your bosses told you. You cannot. Trust is the only thing you have in terms of respect from your coworkers. Don't throw it away. They know it sucks. They know you know it sucks. You don't have to rile them up and pour gasoline into the fire, but don't sugar coat it any more than you officially have to to comply with the bosses request.

There does exist a risky option, that if you have a leader that frequently pivots to new things, or people cycle in and out of that position every few years, then this may be viable:

Ignore, Delay & Bypass

I witnessed something similar to this situation where an executive was pushing a bad decision, and the people several levels below who would be implementing it were killing over laughing at how absurd it was. So with their immediate leader's support they ignored it.

When higher ups asked them for status on their integration of it, they seemed to always have very high priority tasks from customers coming up that were delaying their availability to implement the changes, or they could not risk making any changes while they were in the middle of a critical deployment. It was impressive watching said lead's ability to come up with excuses to kick down the responsibility down the road by a month or two.

Eventually the executive moved on to the next initiative and thus no one was pushing or checking on that bad decision. Other groups in the organization quickly reverted back, mean while their group continued on as nothing had happened.

One key aspect of this strategy is delaying. Even if you eventually have to come in line with a decision, you can take lessons from other groups and mitigate some of the negatives. Also if other groups go through the painful transition first and leadership can see the fallout of the decision they may relent before your group is forced.

Then the last aspect is bypassing. A former coworker once had a requirement pushed upon them that thou shall use a specific framework. Problem was said framework was never intended for said product. So they used it for small portion while using a different framework that actually did the job. I will note this did carry a huge additional drawback: The leadership thought their decision worked and did not learn from their mistake and thus were likely in the future to do the same thing again.