The return details is a non-obligatory field (no asterisk), but as Lloyd says it MIGHT be your airline that demands to see your return ticket. There's no fixed rule. I came in one-way ten days ago and the airline did NOT ask about my return ticket but they DID demand to see the TDAC.
Take fair warning from others' replies that it may be an uphill struggle for you if your potential employer is so completely clueless about employing a foreigner. The idea that they sent you to your own embassy to ask about working in Thailand is embarrassing.
- Brilliant, Monsieur Poirot, but I'm capable of remembering which passport I'm travelling on. Entry stamps are a great help with that 🤨. All passports used are in the system, and they are free to ask to see the other one/s.
Be aware.. they are aware of your other passport/s. I also have more than one passport and I have often been asked to show others on entry (in Thailand and several other Asian countries).
As a general note, in a country with so many visa types, entry types, and exemptions, play your part and physically show the IO what you want to enter on. Sometimes people have a visa in their passport and don't want to use it yet. Others will be assumed exempt unless shown otherwise. Eliminate the guessing games - show your visa.