How to select pool based on http payload ...

You could also use findstr to get the value:

set id [findstr [URI::decode [HTTP::payload]] "\"idTerminal\":\"" 14 "\""]    

This will find the first instance of "idTerminal":" in the payload, skip 14 characters (the length of that string), and then collect everything up to but not including the very next double quote, which in this case should be the ID value.

One thing I'd want to point out here, is that for this method to be used to path management, you have to make sure that either a) a POST payload ID is in EVERY request, b) you have something in place for when it isn't, or c) you configure so,e form of persistence.