Is it permissible to accept a prize in a competition organized by an usurious investment website, knowing that the prize is not cash but an investment plan within the website that will later multiply in value?

It is forbidden to create an advertising video for an interest-based website because it involves assisting in sin. The Prophet, peace and blessings be upon him, cursed the one who consumes usury, the one who pays it, its scribe, and its two witnesses. The prize earned from such an act is unlawful wealth that must be disposed of by spending it on public interests and acts of charity, along with repenting to Allah Almighty. It is not obligatory to return it to its owner. And whoever gives up something for the sake of Allah, Allah will compensate him with something better.