REST service call from APEX application

I am trying to call a REST service from APEX application, where I need to replace parameters in url with values entered in the form.

I know how to do it if the REST service takes JSON as an input but not sure how to replace within URL.

for example the REST service has a POST endpoint as following

http://abc.com/test/val1/val2

I need to replace val1 and val2 in the url when calling the service, by passing form element values.

Thanks for your help.