If you're using MPASM, to set an ID for a 14-bit core, you would insert
@ __idlocs 0x1234 in your code.

With the PM assembler the format is @ ID 04030201h.

Both would set the ID to 1234. For 18F parts it's different, but there's
a good explanation of how this works in the MPASM help file.

Look in PM.TXT for how it works with the PM assembler.