While what Karl writes is true, I think the specific thing you are trying to achieve is not possible, because there are 2 variables for each element type: the ID and the Floor ID.
Additionally, in this format:
WAL-001-FLR1
when new instances are created, the last number is incremented, so new instances will get these IDs:
WAL-001-FLR2
WAL-001-FLR3
WAL-001-FLR4
etc.
I suggest that you use Expression-defined Properties to achieve the results you are after.
Use Karl's suggestions, but with the following format:
WAL-001
Then, new instances will have the IDs:
WAL-002
WAL-003
WAL-004
etc.
Then, in the Property Manager, create a new Expression-defined Property of "Script" type, and compose it by concatenating the Element ID, the "FLR" string, and the Home Story Number. Then use that Property in Labels and in Schedules, or even map it to IFC Properties if needed.
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