To Set Focus on Form control in Ax 2012 you can take help from below code.
Args args;
FormRun _formRun;
FormControl _formControl;
;
args = new Args(formStr(CustTable));
_formRun = classFactory.formRunClass(args);
_formRun.run();
_formRun.detach();
_formControl = _formRun.design().controlName(identifierStr(Desc));
_formControl.setFocus();