I need to add ad_group id to ad's URL. However, I can only obtain the id after creating the ad, not before. Is it possible for me to specify a parameter in the URL so that it can be populated by FB when the ad is created? I know double click of Google supports this.
No, there is no way to automatically add the adgroup ID to the referral params of an adgroup - typically developers use their own system's reference for tracking purposes in the url tags, or add the creative ID, something like that