As @erionm indicated you can link purchased MBs from within MDE under the Account section.
If you purchase MBs at the resort gift shop or at a park gift shop for example the CM can link it at check-out for you or leave it unlinked if you are purchasing as a gift for someone else. Just let the CM know.
Each MB has a unique number to allow it to be registered to an individual. Once linked a MB can't be reassigned.