It requires the following features: p:http-request.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:http-request href="http://localhost:8246/service/echo" method="post">
<p:with-input>
<p:inline content-type="application/json">{{"key1" : "value1", "key2": "value2"}}</p:inline>
</p:with-input>
</p:http-request>
<p:identity>
<p:with-input>
<result>
<base-uri>{p:document-property(., 'base-uri')}</base-uri>
<content-type>{p:document-property(., 'content-type')}</content-type>
</result>
</p:with-input>
</p:identity>
</p:declare-step>
Schematron validation
<s:schema xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:pattern>
<s:rule context="/">
<s:assert test="result">The root element is not 'result'.</s:assert>
<s:assert test="result/base-uri='http://localhost:8246/service/echo'">The URI is not right.</s:assert>
<s:assert test="result/content-type='application/json'">The content type is not right.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
