I thought I caught another thread comment on this as NO, you can only do it after your first action.
But looking at the latest v1.4 FAQ, the runners 1.1 allows for paid abilities to be triggered. Such as a self modifying code?
Or maybe it was something like you couldnt take your "when turn begins" credits if applicable, and then use those to pop Self-Modifying Code. That would make sense as per runner timings 1.1, 1.2 and 1.3
So basically if you have the cash im assuming its fine to pop before any clicks. If not, and the start turn credits would have done it. Too late. You gotta take an action.
You've got it. All of your start of turn stuff happens in 1.2 (Aesop's, Hard At Work, etc.) You can fire your SMC at 1.1. You can also (and generally probably should) fire it at section 3.2 of the corporations turn for the reasons that other people said (generally to get your kate discount) You'd get the parasite token at 1.2 if you did it on your opponents turn or not.