Here's a better idea. A feature was added to FileMaker to specifically deal with this issue.
Define a text field with this auto-enter calculation:
Get ( UUID )
Use this field as your primary key. The Uniquie Universal ID code generated by this function is intended to be unique across all tables and devices.
Wow! That's pretty cool. Thanks PhilModJunk.
However, I really need to limit the length of the assigned value to 15 characters. We use this value, sort of like a phone #, to quickly lookup a customer. Having to type the string producted by Get(UUID) would be pretty messy for a quick search by ID. It would also require changes to the structure of our master database to accomodate such a long string value in both parent and child tables that use this field in relationships.
Is there no other possibility?
Of course there are, but what I suggested is what you should use for your primary key. Don't use it for looking up customers. What you describe is an ID field, but not one that should be used as the primary key to link records in a relationship.
An ID that starts with part of a user's account name followed by a serial number doesn't strike me as a practical ID for looking up records. A name search that can also filter by an auto-entered account name sounds much more user friendly and there are a number of ways that you can set that up for finding records.
To get the ID that you want here, use two fields. Define one as an auto-entered serial number and then define a text field to auto-enter the serial number field's value combined with the text that you want in an auto-enter calculation.