Dear Stephane,
you can use a post like this (tested with postman):
First Login: http://docuware/DocuWare/Platform/Account/Logon?UserName=xxx&Password=xx...
Then a POST command (with curl - postman or other tools)
http://docuware/DocuWare/Platform/FileCabinets/c664c5fd-6ae4-48f6-bd2e-9ac81628089a/Documents/57/Fields/
With xml (or Json) as body, which contains index fields:
http://dev.docuware.com/schema/public/services" xmlns="http://dev.docuware.com/schema/public/services/platform">
02
Kind Regards