According to:   https://en.wikipedia.org/wiki/Percent-encoding#The_application.2Fx-www-form-urlencoded_type

.. your target server only accepts old-fashioned name-value-pair form data, not JSON.

Essentially create a query string separating name=value pairs using '&" then url-encode the entire string?

See https://deltacloud.apache.org/curl-examples.html

Launch an instance. Note that in this example parameters are specified as application/x-www-form-urlencoded through the use of the -d flag:

 curl -X POST --user "user:pass" -d "image_id=ami-84db39ed&hwp_id=m1.small&keyname=marios_key"  http://localhost:3001/api/instances?format=xml