Calling nlapiSetRedirectURL from a suitelet after creating Form

I have a suitelet that when called creates a form using the nlapiCreateForm object. In the form I am adding a button that calls a function, within this function I call nlapiSetRedirectURL.

e.g.

function initiateForm(request, response)  {          if ( request.getMethod() == 'GET' )       {               form.addButton('custombuttonpayment', 'Add Payment', 'addpayment()');         }       else      {         }  }    function addpayment()  {        // do some stuff        submitPayment();  }    function submitPayment()  {         nlapiSetRedirectURL('SUITELET',3, 1, null, params);  }

For some reason

nlapiSetRedirectURL('SUITELET',3, 1, null, params);

is not being executed. I can executed the nlapiSetRedirectURL succesfully in POST part of the getMethod function but cannot get it to execute in a function outside the main function initiateForm.