Pressing the Return key merely clicks the closest button based on several factors (number of buttons, order of declaration, whether or not a Submit button exists, etc.), and is no guarantee of performing any specific action.
You didn’t mention the action you wish the text box to perform, but generally, when you want to trigger a specific action related to an input field like a textbox, you either code one of the textbox’s action events to call a Javascript method, or the form’s onSubmit attribute to call a Javascript function.
Without more specifics, I assume you want to do something with the text before the form is submitted? If so, do you want to cancel the submission if certain conditions are not met?
It would help if you fully describe the sequence of events you wish to occur.