nable to create mocks on post request with content-type : application/x-www-form-urlencoded

I am working with mountebank to create mocks for an External REST API which is a POST request with content-type : application/x-www-form-urlencoded.

The API is of format
https:///dpay/MPmt

and data payload is passed in format msg=01223~20170607114321~ABC~12345~NA~NA

I have to search on the basis of ~ABC~12345~ which would remain constant.

I have used contains, matches and equals predicates but was not able to run them while passing the payload in request body.

However, I was able to get it running when passing payload as a query parameter i.e https://url/dpay/MPmt?msg=01223~20170607114321~ABC~12345~NA~NA , but can't find a way to get it done when passed in request body.

Any pointers would be really appreciated.