I have done over the counter transfer of funds from other non Bangkok bank branches into my Bualuang trading account. I used a copy of a deposit form that was provided to me by BLS. I recall the form just has the details of your BLS account number on it. You should email your BLS broker and ask them.
If you are transferring from abroad more than the equivalent of 50K US into a Thai account, it is wise to obtain a Foreign Exchange Transaction (FET) (sometimes referred to as Tor Tor 3) from your bank. That will make things less problematic when you wish to transfer funds back out of Thailand.