Hello,
to create a reaction with a modifier (be it activator, inhibitor or otherwise), that is a species partaking of a reaction that is neither consumed or produced by the reaction, you enter it at the end of a reaction scheme separated by a semicolon. For example:
A -> B; Inhibitor
This defines a reaction where A is converted to B, with an additional species Inhibitor. Then you select a predefined kinetic making sure that inhibitor is mapped to a modifier.
if you have a specific example in mind feel free to share and we can look at it together.
best
Frank