Here is some code example

<form name="WWW" id="WWW" action="/csp/foo.cls" method="POST">
    <textarea id="PARAM1" name="PARAM1"></textarea>
</form>
document.WWW.submit();

Which will result in the following request :

POST https://something/csp/foo.cls?PARAM1=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX HTTP/1.1
Accept: */*
Content-Type: text/html;charset=UTF-8
Referer: https://something/
Content-Length: 0
Connection: Keep-Alive
Cache-Control: no-cache

As you can see, input value is added to the url, just like a GET.

I found the following XSD file which seems to be what I want :

C:\InterSystems\Cache\bin\cacheexport.xsd

It's much more complex than what I thought. Maybe exporting all cache files as UDL (as Dmitriy suggested) is a better approach. I don't know if SyncTool is able to export cache entities in that format directly. If not, I will need a second pass (that convert xml to udl).