Hi @hector_borras,
There are a couple things to note when considering Tweeting on behalf of advertisers.
If you use the POST statuses/update endpoint in our REST API, you’ll only be able to Tweet as whichever user your OAuth token is associated to.
If you have been granted Tweet delegation status via ads.twitter.com, you can use the POST accounts/:account_id/tweet endpoint in the Ads API to Tweet on behalf of the advertiser’s FULL promotable user. To check whether you have been granted to TWEET_COMPOSER privilege for the user associated with your OAuth token, use the authenticated_user_access endpoint.
To know the user_id that you have permission to Tweet on behalf of, check for the FULL promotable user on the ads account, using the promotable_users endpoint.