Hi @OP_11714882,
In ThingWorx, there are 2 ways you can call Windchill REST API.
First is to create a Thing Service and use ContentLoaderFunctions.
Refer to articles:
https://www.ptc.com/en/support/article/cs237875
https://www.ptc.com/en/support/article/CS380751
Second is to use Integration Connector > ODataConnector
Refer to ThingWorx Help Center
As for which Windchill API to use to create WTDocument, I found below articles, but if you have any further questions, you need to ask in PLM community board or open a Windchill support case for help.
https://www.ptc.com/en/support/article/CS370202
https://www.ptc.com/en/support/article/CS363846
Hope that helps.
Best,