return value of msgbox

I have a form and when submitted it will first check some values with VBScript.
Then a msgbox will pop-up asking to submit the form (Yes or No).

It is so that which button i click the form is always submitted.

I have the following sort of function:

FUNCTION MyForm_OnSubmit
  msgbox("Continue?"), 4
  MyForm_OnSubmit = TRUE
END FUNCTION

Now i want to submit the form when pressed YES in the message box and not when pressed NO in the messagebox. How do i do that?