there is a public class method to add field to mechanize form
I tried ..
#login_form.field.new(\'auth_login\',\'Login\') #login_form.field.new(\'auth_lo
another way how to add new field is to so at the time of posting the form
page = agent.post( url, {'auth_username'=>'myusername', #existing field 'auth_password'=>'mypassword', #existing field 'auth_login'=>'Login'}) #new field