Add activity to deal's owner

How can I add an activity to a deal using the API with the activity owner being the same as the deal owner, without knowing who is the deal’s owner, in 1 API call ?

Unfortunately I can’t think of any way to do this within 1 call if you don’t already know the Deal owner.

Then please allow it be setting the ownerID to a predefined value

How should I do it now ?

You would first need to use GET/deals/{id} to find out who the Owner of the Deal is, and then their User ID to the user_ID paramater of POST/activities