Changing The Password

i am doing a computing project using access 2003 and vba.
i have designed 2 forms A and B, form A is where a user to logs in to my system (with four main items: username (combo box), password (textbox) and two buttons) and form B is where the user can change their password (with four items: username (username (combo box), new password (textbox) and password verify (textbox) and a button that checks everything and updates the password).
Form A has two buttons, one button can log the user into the system after typing in their correct username and password credentials, and the other button gives the option to change their password also after typing in their correct username and password credentials (and the opens up form B for the user to change their password)
My question is, when the user presses the button to open up form B, what code could i use (or methods in access) that would set the username field (in form B) the same as in form A automatically without the user having to select it again?
All help would be much appreciated