@Elizma ******
Then ask your airline if they will require a flight ticket departing Thailand within the 29 DAYS FROM YOUR ARRIVAL DATE INTO THAILAND?
Remember that the Day you arrive into Thailand COUNTS AS DAY #1.
IF they will require one... Book a cheap flight ✈️ ticket departing Don Mueang airport in Bangkok to Phnom Penh. EST cost of US 40. One way.
IF you can book it and cancel it Free within 24 hours... print out the booking and cancel the ticket.
Search 🔍 Google.. your friend for flights information.