My knowledge on this topic is a bit rusty. But this should be it:
License activation is briefly documented at https://github.com/mendix/cf-mendix-buildpack#license-activation
For the activation you need 2 strings:
The server ID you are referring to is the hash value of $LICENSE_ID ; I’m not sure if I’m allowed to disclose exactly how this Server ID is computed. The formula is actually very easy to derive. According to docs https://docs.mendix.com/developerportal/deploy/licensing-apps-outside-mxcloud#3-6-docker it’s better to raise a ticket with Mendix support so that they can generate both the LICENSE_ID and LICENSE_KEY for you.
An alternative (hardcore) way to get a hold of server id is:
The alternative method is all pure public knowledge.