Well I tryed out hiding Edit, Save and Cancel buttons in newForm tab, them added a footer with buttons and wrote their own JS managing field updates and firing spjs.dffs.check_Mandatory(); and spjs.dffs.triggerSave(); worked like a charm
Hope it help others
function NameOfFunction() {
spjs.dffs.alert({
"title": "Title",
"msg": "Message text",
"ok":function(){
var allReqFieldsFilled = spjs.dffs.check_Mandatory();
if (!allReqFieldsFilled) {
spjs.dffs.alert({
"title": "Title",
"msg": "Message text",
"ok": function(){
spjs.dffs.closeDlg();
},
"okBtnLabel":"Button Name",
});
}
else {
setFieldValue("FieldName", "Value"); //Dropdown options
spjs.dffs.triggerSave();
},
"okBtnLabel":"Button label",
"cancel":function(){
spjs.dffs.closeDlg();
},
"cancelBtnLabel":"Button Label"
});
}