In VOLT, you put the substitution string in the top part of the lookup window, and the context in the lower part, on the left or right of the | depending on whether the context is before or after the glyph(s) to be substituted. [Note that left/right refer in the contexts to before/after, and are not directional, i.e. for a RTL script like Hebrew, the before context still sits to the left of the | marker.]

I presume you are wishing to substitute qamats qatan for the regular qamats when it follows (?) maqaf. If so the lookup would be like this:

qamats -> qamats_qatan
maqaf |