I have an item with a serial number — say 1234.

I want to create a number of sub items — say three — with serial numbers 1234-1, 1234-2 and 1234-3.

I can’t seem to figure out how to do this.

How important is it to follow this exact format? You could easily number the sub-items (presumably in their own child table) with their own serial number and combine the two serials so that you would end up with say:

1234-1
1234-2
1234-3
1235-4
1235-5
1236-6
...

Otherwise you would need to script the process or at least a part of it.

P.S. Resist the temptation to rely on counting sibling records. It can easily lead to producing duplicates when two users create records in the same time frame.