Any reason why? You can’t do anything with Unity off the main thread, so it seems reasonable to me.
Here’s my blurb about the typical marshal-to-main solution:
Delegate queue to marshal tasks from other threads to the main thread:
To debug anything related to Networking, UnityWebRequest, WWW, Postman, curl, WebAPI, etc:
And setting up a proxy can be very helpful too, in order to compare traffic: