A default value of "0" in a inputfield box?

Hi folks,

Can you help me please? I have created a form in flash and like in VB there are properties to set a NULL value as default. Can anyone help me to tell me what bit of actionscript I need to use to do this and where do I hold the variable?

So that in my form is the user deletes the value out the inputfield and hits submit it still leaves a value of 0 in the box?

Thanks for any help guys!