Postfix-Definition

 Clear@"`*"

 Sin*Cos // f[r_] := Sin[r]*Cos[r]
 
 Sin*Cos // f[Pi/7]

 (* out *)
 Cos[Pi/7] Sin[Pi/7]

Prefix-Definition

 Clear@"`*"
 g[r_][Sin*Cos] := Sin[r]*Cos[r]
 
 g[Pi/11][Sin*Cos]

 (*out*)
 Cos[Pi/11] Sin[Pi/11]

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