Pre populate another list new form when drop down value changes in current list form

Hi, 

Use a JavaScript Modal Dialog containing a frame where the URL is the "New Item" one of list B.
And let this Modal appearing once you select a certain value from the Drop down.

this might help with what Philip Warrak‌ has suggested?

https://community.nintex.com/community/dev-talk/blog/2017/02/02/ensure-unique-reference-on-new-item

I found out the way to do this. It's working fine now. Thank you for the support!.

Excellent - can you detail your approach in case others have the same requirement please?

Let's assume that my drop down ID is MyddlID
NWF$(document).ready(function(){

NWF$("#" +MyddlID).change(function(){
var obj = NWF$('#' + MyddlID + ' option:selected').val()

if(obj=="Y"){
javascript:SP.UI.ModalDialog.showModalDialog({ url:'list B url', title: 'B-New Item' });

}
});
});

And also here I have question: How to hide "Close button on the top-right of the page"?

Dear ‌,

Please take in consideration freezing the screen behind the Modal Dialog.

Regards,