Supply exactly one of brand_id, advertiser_id or a Meta Ads Library URL with view_all_page_id. Advertiser and URL targets select: an already-followed candidate, otherwise the most likely brand, otherwise a brand representing that advertiser. source_advertiser_id is allowed only with brand_id and must belong to the selected brand. Repeated follows do not add a second follow. Quota counts brands. A URL collection request can persist even if the later follow fails; requested ads become available asynchronously. The full result includes followed_at and quota.