Since is retired, you cannot buy a new license key. Instead, the developers recommend:
Where is the Campfire Pro license key? : r/CampfireTechnology campfire pro license key
: A standard license key typically allows for activation on one computer at a time. To move it to a new machine, you must deactivate it on the old one first. Moving from Pro to Campfire Write Since is retired, you cannot buy a new license key