You can use a lookup to do a selection. But you would need to turn the result into a boolean value. For example, if you wanted it to return true if there was any match in the other database, you could use this formula:
lookup("Installed base",«Customer Code»,«Customer Code»,«Customer Code»,"")<>""
Since your key field and data field are the same, simply looking for any match must be what you had in mind.
At the end of the selection, you'll have a list of all records in the current database that also have a corresponding record in the Installed base database.