How do you open a Thai bank account to deposit the 800k before getting the visa? Or do you get the visa first then deposit the money, do I need an agent ?

Hello. I have some questions about 800k THB deposit for non-O retirement visa extension. My wife and I will apply for non-O retirement visa (valid for 90 days, single entry) at the embassy in our country before leaving for Thailand. After entering the country, we plan to open a bank account and do the international fund transfer to the account. Then, apply for 1-year extension in Thailand within 90 days.

My questions are

1. Is it required to provide the proof of international fund transfer for applying visa extension? Or a proof is not necessary and it is okay to bring in cash or bank check?

2. It is obviously not possible to keep the 800k THB deposit in a bank account for 3 months before the application because deposit will be made after entering the country. How can we solve this issue? Any idea?

TIA.