One way to do something like that, is to ensure that your `k` parameter of your Reaction is a global parameter. Then you could target it by an event.
Attached a very simple example:
- global parameter R1_k, initially 0, since we don't want the reaction to proceed
- reaction R1: A -> B, mass action, using a global parameter R1_k
- an event, that triggers the reaction. Here at time > 1, R1_k will be set to 1.
I hope this helps,
best
Frank