EXPORT UNDER LUT

As per circular 40/14/2017 dt.6/4/18, no confirmation is require from deparment , should have the ARN submitted for LUT

can someone from u guys tell me how to get Acknowledgement of LUT. Acknowledgment of LUT is asked by custom officers at Nepal border. I have the application form which is downloaded from gst portal but now they are asking for acknowledgement of application for LUT. They have shown the acknowledgement submitted by another taxpayer as well. How to get the same? please help its on urgent basis.