Leave it blank
You just had reentry permit thread.
https://aseannow.com/topic/1352004-re-entry-permit/#elControls_19593365_menu
Despite your concerns re obtaining at airport perhaps the 200b service fee for filling in TM8 might be an option.
Note: also if obtaining permit at immigration the departure date is irrelevant along with return date.
The permit will expire same as current permission of stay date.