Tamper with post data before it is sent

I am currently doing some pentesting for my work (not my job role but i always wanted to learn). I am trying to modify post and get data as it is sent (intercept, tamper), the web app I am trying to modify appears to be some kind of MVC which means I cannot see some of the data unless I am monitoring the traffic (cannot just inspect element).

I am using Kali Linux, I do also have a windows machine which has hackfox on it but sadly this is an alpha build and it does not function with the hackfox application.

currently using webscarab to monitor the traffic.