Use the gene name plus "antisense" plus a number. Start at 1, and increment if more than one known.

Real example:

FT                   /primary_name="adh1-antisense-1"
FT                   /primary_name="adh1-antisense-2"

If the gene has no standard name, use the systematic ID. No real examples, but a made up one might look like "SPAC99A111.01-antisense-1".