How to create custom form in HTML for Suitlet in NETSUITE and get the form data

/**
 * Module Description
 * 
 * Version    Date            Author           Remarks
 * 1.00       22 Feb 2016     Anurag
 *
 */

/**
 * @param {nlobjRequest} request Request object
 * @param {nlobjResponse} response Response object
 * @returns {Void} Any output is written via response object
 */
function RegisterPageUI(){
   var html=     
     '<form action="" method="post">'
     +'First name:<br>'
     +'<input type="text" name="firstname" value="Mickey"><br>'
     +'Last name:<br>'
     +'<input type="text" name="lastname" value="Mouse"><br><br>'
     +'<input type="submit" value="Submit">'
   +'</form>';
 return html;
}


function Register(request, response)
{
 if(request.getMethod() == 'GET'){
     
   var html = RegisterPageUI();  
   
      response.setContentType('HTMLDOC');
      response.write(html);  
      
     
     
    }else{
     var firstName = request.getParameter("firstname");
     var lastName= request.getParameter("lastname");
          
     response.setContentType('HTMLDOC');
      response.write(firstName + lastName + firstName + lastName+ firstName 

       + lastName+ firstName + lastName+ firstName + lastName);  
     
    }
    
}