The only way to pre-populate the promotion text is to use Interactive Messages instead of a shared link. This is the feature that comes with Google+ Sign-In . This may not be the best solution for you if you are trying to use a generic approach. However, if you are already using Google as an authentication method, then it may be a good opportunity for you to sign in to Google+. Interactive messages allow not only pre-filling the text, but also filling up to 10 recipients of this promotion, and all of them will be directly notified about it.
source share