How can add URL to dropdown items for a profile page
profiles are simply a way to build forms so only a subset of all the gazillion fields that civi have are exposed to the end user.

so the initial question is what is that supporter field you want to add? It has to be already created in civi before you can use it.

that notion of supporter doesn't exist in civi by default, so either you are using an existing field and slightly changing it's meaning, or you need to first create a custom field 'supporter' so it's exactly what you want.

if by link you mean letting the end user putting their website, there is a core field for that- website, that you can simply add to the form