For Registry Alias Files, you must specify a GUID and a Key Root where the Alias File will be located.  You do this by simply entering the information in the controls on the Alias License File Tab of the License Manager as shown:

If you wish, you can use the "generate" buttons beside each field to randomly generate the appropriate field value. For the GUID field, this generates a unique GUID using the standard system creation method. We strongly recommend that you only ever "Generate" the GUID to ensure it does not conflict.

You have the option of choosing one of three main registry hives for the Key Root.  Depending on the hive you choose the Key Path is a set value (from that root) that cannot be changed.  To find the exact path, see Registry Alias Files.

There are important security and permissions considerations that you must take into account when using Registry Alias Files.  Be sure to read Windows Permissions And Security Considerations.