If you wish to define a default location for the User Database that will take effect if the UserDatabaseSpec option (UserDatabaseSpec) is missing, you can define it, for example, like this:

APPENDDEF(`confMAPDEF', `-DNEWDB -DUDB_DEFAULT_SPEC="/var/db/userdb.db"')

The backslashed quotation marks are necessary to pass the path to sendmail as a string.