Hi, @SocialOpinions. Thanks for your question and for providing all of these details.
Your understanding is correct—you do need to use the as_user_id parameter when creating a promoted-only tweet for 18ce53zy2tx. However, you must also specify this user’s account ID (instead of 18ce549rjnj) in your request, as shown below:
$ twurl -X POST -H ads-api.twitter.com "/1/accounts/18ce53zy2tx/tweet?status=TESTING_123&as_user_id=38906681"
The account ID used in Ads API endpoints should belong to the account you’re managing.
Hope this clears things up. Please let us know if this resolves the issue. Thanks!