If I run term_start('zsh', {'term_name': 'MYTER','vertical': v:true}) I get the expected behavior, but if I run term_start('zsh', {'term_name': 'MYTER','belowright': v:true}) will produce E475: Invalid argument: belowright, in-spite from the doc one can read
"vertical" split the window vertically; note that other window position can be defined with command modifiers, such as :belowright.
I am using MacVim 9.0.
The vertical mentioned in the doc is a key in a dictionary. The function knows how to handle that vertical key but it can't handle any arbitrary key like belowright or foobar. In other words, belowright is not part of the type expected by the function so you get an error if you try it.
The command modifiers in question are :help :vertical, :help :belowright, :help :topleft, etc. They are used before a command that creates a new window, to influence how the split is done.
In this case, "the command that creates a new window" is the whole:
:call term_start('zsh', {'term_name': 'MYTER', 'vertical': v:true}) which means that you are supposed to use :belowright like this:
:belowright call term_start('zsh', {'term_name': 'MYTER', 'vertical': v:true}) 
vertical was a pre-defined key of a dictionary and that my attempt was not going to work. I also try the nastier ...'vertical': belowright :D Needless to say it didn't work - it, not surprisingly, complained about the value type associated to the key vertical :D Nevertheless, I solved with botright term_start(... in my vimscript. :)
term_start()in Neovim.