How to award a badge by buying a gamepass?

I have created a badge in an early stage of the game and I would like to sell the badge to players who want the perks, e.g. chat nametag, boosted currency gain.

Assuming that the gamepass ID is 8857115 and the ID of the badge is 2124516381 how can I award the badge if the gamepass is owned by the player when they join the game?

As a new scripter any help would certainly benefit me and I would be immensely happy.