Can I buy or sponsor an offer on a GPT site?

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I am looking to promote a fantasy investment site for referrals. Will the top GPT sites like cashcrate or treasuretrooper allow members to buy/sponsor/advertise(I don't know the right wording) a free paid signup offer? My plan is pay $.50-.75 per signup. If yes, what would some basic requirements? If not, what are some good and preferably inexpensive alternatives?