First thing I noticed were "account number" and (Usually a D or G). This confused me.
From my experience, I would either A. Search in my gmail for the account # or B. find a physical piece which has my account number.
Solutions.
You could have the first two 'letters' already filled out, assuming they are globally ubiquitous. An example would be my Permanent Resident Alien Card which starts with A# -- When I fill out an application that requires this, it's usually pre-populated. Another example would be your driver's license.
have a char counter below your input field e.g. '10 characters remaining'.
When you are asking for an account number, it would need context to why it needs your account number - i would have a link "forgot your account number" and have some requirements to have it sent to users email.