Testopia has a XML-RPC API based on the Bugzilla XML-RPC API. You can read more about the details at https://www.bugzilla.org/docs/5.0/en/html/api/Bugzilla/WebService.html
devZing also provides an online XML-RPC test client at https://xmlrpc.devzing.com/
API Docs:
Example
Here is an example HTTP POST to https://app.devzing.com/testopia/bugzilla/xmlrpc.cgi for the Testopia.testopia_version method.
Content-Type: text/xml <methodCall> <methodName>Testopia.testopia_version</methodName> </methodCall>
Here is the response
<?xml version="1.0" encoding="UTF-8" ?>
<methodResponse>
<params>
<param>
<value>
<double>2.5</double>
</value>
</param>
</params>
</methodResponse>
Example 2
This example includes authentication and passes a parameter. The method Build.get retrieves the specific build.
<methodCall>
<methodName>Build.get</methodName>
<params>
<param>
<struct>
<member>
<name>Bugzilla_login</name>
<value>demo@devzing.com</value>
</member>
<member>
<name>Bugzilla_password</name>
<value>password</value>
</member>
<member>
<name>id</name>
<value>
<int>1</int>
</value>
</member>
</struct>
</param>
</params>
</methodCall>