I mean that 'add_action_buttons' does not have $attributes! -> https://docs.moodle.org/dev/lib/formslib.php_Form_Definition#add_action_buttons.28.24cancel_.3D_true.2C_.24submitlabel.3Dnull.29.3B
Why can't you just use the CSS selector:
#page-login-signup [name="submitbutton"]