Thailand reader question: Can someone give me information about that Pre-approval

You started at the back instead of the front. After all, a COE is what you need to enter Thailand.
Your first step is to make sure you have the right visa, most people say visa. Then you can start with the application.
\click the following https://coethailand.mfa.go.th/regis/step?language=en

and you start the road to COE, (Certificate of entry) so your entry ticket to Thailand.
You will then enter some standard data. Passport, to etc. All fairly straightforward.
Please note make sure you have all the necessary papers at hand. Including insurance, etc. But, if you don't have that, don't worry. If you come across something that you don't have available, you can just close it, look up the relevant part or something and just start again.
If this part has been done satisfactorily, you will receive a message from the embassy after a few days that you have a preappovel. Then you have 15 days to book a plane ticket, hotel, insurance. With the notification of pre-approve you will receive a number with which you can log in each time. So even if you don't have something yet, just log out and search for it.
Couple of comments. You are asked for your name (surname) and first names. The latter sometimes goes a little wrong. Must be exactly as stated in passport. But sometimes there are many first names in passport. Then when you log in again you may sometimes have to make a few attempts to get into it. If this happens to you: discipline !Write down what you entered each time you try. Finally you see what people expect.
The embassy in the Netherlands only wants to start with your application from 1 month before you want to leave. And, as mentioned, after the preappoval, you have 15 days to book the insurance, hotel and flight. Good luck.