Without custom development, you can't. Its not something that is automagically built in, it (webhook criteria) has to be developed for each webhook individually as is considered an advanced feature, not basic feature.
I have not got to the point where I can start developing advanced webhook features such as Category Choosers and Blog Choosers like the Thread webhook has for Node Choosers. XFRM and XFMG have not developed any advanced features like Category Choosers yet either (I am sure they will eventually tho).
Anyway... it is planned