How do I get a get a text from a developer product?

No idea how to explain it.

If a player buys a developer product and they want to change their licence plate. When they pay for it how will the server know what they wanted to change it to?

Player enters - LICENCE123

Player buys dev product

How will server know that the player wanted licence to be LICENCE123?

Well, it's quite basic. Just have an identifier or a table.

local IDs = { [ProductId] = "LICENSE123"; [ProductId] = "LICENSE345"; };

Etc,.