Where does this domain come into play? It’s not clear what the use of this field is, which makes it difficult to understand what we should fill-in here.
As per the docs for this “sometimes required” field:
This setting is available for line items using the TWEET_ENGAGEMENT objective on the Twitter Audience Platform (TAP).
advertiser_domain specifies the top level domain corresponding to the advertiser whose ad you are showing. This is for black-listing content by the publishers.