realtime POST interceptor and sender based on filters

Question

I'm looking for a way to intercept a POST method to a website and change it's content based on rules.

Given example:

A website is sending a data along with temporary unique identifier. I want to edit only the data I want, leaving the pre-generated identifier as-is.

Example POST:

ageStart=1&ageEnd=19&regions=101&navigator=Near&consent=BOey6KLOey6KLA9ABAPLCN-AAAAmd7

I wish to edit the ageStart and ageEnd variables without touching consent variable which is different every time POST is forged

I'm looking for either the software or a Firefox/Chrome extension

Answer 1

I've found Fiddler to be enough.

First I opened the software, enabled HTTPS decoding, then enabled breakpoint BEFORE request and easily edited the variable.

Everything went correctly and the server saved data with malformed data without touching the temporary identificator.