How can I contact the embassy for an extension on my visa application process?

Why would you contact the Immigration, they have nothing to do with issuing visa’s.

That is handled by Embassies.

what documents „from your

home country“ will take 3 weeks? 😅

At this stage, you're dealing with the embassy; immigration has nothing to do with it until you cross the border.

I would try to reach them by phone. They will absolutely auto-reject the application in 14 days and keep the fee

You don't contact any immigration office. You don't apply for visas through an immigration office. That's a completely different government agency.

Visas are applied for through embassies. So you contact the embassy, not immigration. You should have an application number. Usually the best way to contact many Thai embassies is through Facebook messenger. Try to find if the embassy you applied through has a Facebook, and send them a very concise message with all information in a single message. What your name and passport number is, what your application number is the visa you are applying for, and why you need to message them. If you make it a long message, or send it in multiple messages, it will likely be ignored.

If they don't have a Facebook, you can try to find a phone number or e-mail address, but both of those are nearly useless for contacting most Thai embassies.

Upload a letter that explains why it’s taking soo long