@Brett **********
Having a FET is useful to have as it proves where the money came from if and when you attempt to move the money out of Thailand. There is a threshold of I believe $50,000 under which FETs are not typically issued. For immigration purposes, immigration officials look for indications in your bank passbook and statements that the money came from an international transfer. For example, in the case of Bangkok Bank, your passbook transaction will be coded as FET (Foreign Electronic Transfer). Immigration understands what these codes mean and looks for them during your application process.
Yes, you can use TransferWise to do the international transfer but you must take care to do it correctly. How to do it correctly is spelled out in this group
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but basically, it requires that you use Bangkok Bank as your Thai bank and that you indicate on the TransferWise transfer that you are doing it for the purpose of longterm stay in Thailand. This will result in the correct FET code being applied to your transfer. Using another Thai bank or failing to indicate the long-term stay in Thailand reason is likely to result in a transfer marked other than FET and which immigration will not accept.