This is untested, but from a quick read of the docs you linked to, I believe the best setup would be an action with two steps as follows…
Step 1: Callback URL
Configure the callback url step with the following URL and “Wait for response” enabled (replacing “uppercase” with the action you want to run:
cleantext://x-callback-url/run?action=uppercase&text=[[draft]]
Step 2: URL
URL step configured with Drafts’ own create URL, and the result returned by Clean Text:
drafts5://create?text=[[callback_result]]
The callback URL step will create the [[callback_result]] tag to contain the result parameter returned by Clean Text.
This could be done other ways, but this seems the most straight forward.