You must already have a paid Plesk license on your source server.
Offers the Plesk Web Host Edition for free with certain dedicated server plans. plesk free licence key
Includes the Web Admin Edition for free on all Cloud VPS plans, allowing management of up to 10 domains. You must already have a paid Plesk license
Submit a request through the Plesk Licensing Support form with your current license details and the IP of the new target server. 4. Comparison of Plesk Editions plesk free licence key