This would be seen as gambling and unless licenced or within specific criteria is likely to be illegal.
https://www.gamblingcommission.gov.uk/PDF/Organising-small-lotteries.pdf
A prize competition would be legal, but it would be up to the forum admins/moderators as to whether it complied with forum rules
Bonus ball lotteries donβt comply with prize competition requirements
https://www.gamblingcommission.gov.uk/for-the-public/Safer-gambling/Consumer-guides/Running-prize-competitions-and-free-draws.aspx