When doing a REST POST call there is the option to manually set the body to content required.  Usually this requires formatting it if using form values in a calculated field.  This calculated field may contain XML/JSON/Text etc.

In the REST data action remember to set the additional setting Method type to POST.  Then using an input param for calculated field of PostRawBody or PostRawBodyURLEncoded if you need it to be encoded.

Finally set the additional setting PostRawContentType to the XML/JSON/URLEncoded type etc.