Ninja forms Unique ID generation

So i have been using ninjaforms for quite a few times and even though its easy and helpful, somethings were difficult for me to figure out.–

i want to have a unique ID field. which simply means that there should be a unique ID (request number) that gets sent to the user who submits the form and that UID also gets stored within submissions for future references.. i tried many things actually. i know a UID can be made by using eg. Hj28nQ[nf_sub_seq_num]kLo7 within the email. because every submission number is unique, its helpful. but it doesnt get stored in submissions (bcoz it gets generated after the submission as i was told by customer care).
i have tried number of ways..
also tried to set default value of a text field.. by default all we can set is ID, name, email, and some other post related info.

in ‘custom’ default value, i tried putting _user_id and it worked.. but when i tried to add combination of words around it, it was printing as it is.

i just want those who submitting to get some Unique ID and submission is important for future cross reference…

PS- i also thought that maybe if i add some custom field in wordpress itself and grab it using ninjaforms like how it grabs name and all then it might help, but it was way too difficult to design a plugin for that( couldnt get past how to make the field unique).

Uniqueness is important.. thank you, sorry if difficult to understand.
am not sure how querystring varibale option is used.