Note that this post is complete conjecture and could be coded differently.
If flash is anything like java, an object must have the focus to be able to accept key events like enter and typing. Therefore, if you're typing in the password field, the password field has the focus. If you're typing in the username field, that text field has the focus. Note that buttons are different since it's a click event.
Tab switches what the focus is to the next area. Clicking something with the mouse also gives that object the focus if it is set to be focusable.
It IS possible to set the focus within the program. Like if a button is focusable and you click on it, it has the focus so doing anything with the keyboard will result in nothing happening, but in the area where you put the code for what to do when the button is pressed, you can add a line of code that changes the focus.
For instance, let's say you have a game involving the arrow keys and a button to change the difficulty. If the button is focusable, clicking the button means that the game won't respond to the arrow keys unless you click on the game again. However, to solve this, you can either set the button to not be focusable, or add a line of code to change the focus back to the game after the button does its job.
The problem with this is when we get to password / username fields and then the done button.
If the password field has the focus, in now way will pressing enter activate the done button. If the username field has the focus, in no way will pressing enter activate the done button. The only way to make enter activate the done button is to give it the focus, which can't be done without pressing tab (adding a line of code to give the done button the focus when you're done typing wouldn't work because the game doesn't know you're done typing until you press the button).
AKA, It can't be done, if flash is akin to java and Zanz coded it as I think he did.
...Thinking about it, you could add a key listener to the password field object and make it perform the same code that the done button does when you press enter while focused on it, but depending on the code clicking the done button performs, that could make it longer to load. I dunno.