Set field to Mandatory in custom form is Suitelet

Hello All,

I am creating a custom form using the nlapiCreateForm('formname') object and creating a form field. Depending  on certain conditions I am making some of the fields mandatory, e.g.

form.addField('accountnumber','text', 'Account Number').setMandatory(true);

What i need to do is first of all set the field e.g.

form.addField('accountnumber','text', 'Account Number');

then later on in my script i may need to set it to mandatory, How do I set a field to mandatory that has already been created to the form?

Thanks