I did my transfer using Wise. You need to select the reason for transfer, 'funds for long term stay Thailand'. Bangkok Bank were happy to give me the letter confirming the funds and origin outside Thailand.
I very much doubt you bought a Visa on Arrival when you arrived into Thailand from the USA. I think you arrived under the 30 day Visa Exemption scheme. In which case, you cannot use Chaeng Wathanna Immigration Office in Bangkok but should go to the Muang Thong Thani Immigration Office.
and if that is the case, covid extension applicants would tick the box 'my embassy does not give covid letters' and include a print off of the reply confirming that.
says above, or email your embassy asking for the letter, then print off their reply saying they don't issue the covid letter, and tick the appropriate box on the form, my embassy doesn't issue a letter.