The link is the least valuable part
Forum links are nofollow or ugc almost without exception, so they pass no PageRank. If the link were the point, this service would not be worth buying, and the companies selling it by the thousand are selling something worthless.
What is worth buying is presence: a useful answer in a thread that ranks for a question your buyers ask keeps sending qualified people for years. We measure this in referral traffic and enquiries, not link counts.