If I remember well, since I opened both THB and USD account at the same time, by handing them over a couple of thousand baht, they converted some of it into USD and they opened both accounts without me requiring to do anything else.
Around 5 years ago one of the employee at the same branch, told me that the plan of having online banking transfer of USD going out of Thailand was on the pipeline and soon was going to be implemented for all the FCD holders. So no more trips to the bank spending 20 minutes to fill in forms should one require to transfer out of the country. Still waiting though! That would be putting BBL to a similar level of Western banks in terms of foreign cash transactions.