Form Callback Not Working

I am trying to create a form and the callback function doesn't seem to be working (see below). I keep getting an error msg : (2,779)   Method <FORM>.XXX() not found

What is wrong with it ?

Note : I load and show the form by :

$m 'C:\ABC\LabelForm.pmlfrm'
show !!Labelform

Button  .GensecTog at x1 ymax+0.2 | All / None | callback | !this.xxx() | width 10

define method .xxx()

    $P 'XXX'

endmethod

Any help is appreciated