This is probably not the appropriate forum for technical questions about computer algebra systems, but to restrict representation R to subgroup H

RH := map< H->Codomain(R) | x :-> R(x) >;

should work.

If your representation R is of type Map (which it will be if you defined it as Representation(M) for a G-module M), then to restrict R to subgroup H

RH := map< H->Codomain(R) | x :-> R(x) >;

should work.

If you have defined R as a group homomorphism G -> GL(n,K) for some field K, then you could instead use

RH := hom< H->Codomain(R) | x :-> R(x) >;