Its API reference page says:
" For developer product purchase prompts, connect to
MarketplaceService.PromptProductPurchaseFinished."
But if you go to the link, it says it’s deprecated.
“This event has been superseded by MarketPlaceService/PromptPurchaseFinished which should be used in all new work.”
That’s not really a suggestion. PromptGamePassPurchaseFinished fires for Game Pass assets, PromptProductPurchaseFinished fires for Developer Products and PromptPurchaseFinished fires for all other assets. PromptProductPurchaseFinished is only marked as deprecated to discourage developers from using it over ProcessReceipt, the proper way to process developer products. Check the full context of both points carefully.
In addition to this, a documentation request regarding this has already been posted before which received an answer from a member of the Information Experience Team, specifically the one who wrote the documentation for the prompt finished events. Please do make sure to search before creating a post (request).