The lead fill value is a mask therefore entering 'A' is unlikely to have any effect unless your source value is Empty. If your Account Number is always 6 digits then you could have a leading fill value of AAAAAAA which would ensure an A was pre-pended to the account number. However if your account number is not a consistent length then it would be better to either:
- Add another Account source dimension to your data source and give it a static value of A
- Add a complex expression which would prepend A to the start of all Account codes