How to weld parts to a player when they spawn in?

I know how to use weld and cframe, I just don't know how to make it so that a character spawns into the game with that. For example, I made a model of a backpack through blender and imported it into roblox. I know how to weld it onto the players back, I just don't know how to make a character spawn into the game with that backpack on.