The following does not seem to work:

ex1:=ABC; asym(_, $A,B,C$);

Also with indices,

ex1:= A_{ab} C_{d}; asym(_, $_{a}, _{b}, _{d}$);

It does if you write the product A B C with spaces, and add the required dollar signs, so

A B C

ex:= A B C; asym(_, $A, B, C$);

Ditto for indices; do not write A_{ab} but use A_{a b}:

A_{ab}

A_{a b}

ex:= A_{a b} C_{d}; asym(_, $_{a}, _{b}, _{d}$);