How can I fetch some fields based on a selection from a form (FormA - normal form) and show them in a Add note field inside anther form (FormB - stateless form)? 

The story is this:

FormA = contain fields: User_acc (single line), User_link (single line)

FormB = contains fields: User_acc (single line), Note (single line)

In FormA is saved all the links (User_link) associated with a User account (User_acc) ... so we will have a database like this:

User1 | Link1

User1 | Link2

User1 | Link3

User2 | Link11

User2 | Link6

User2 | Link33

User3 | Link5

User3 | Link2

User3 | Link4

Now in stateless FormB, I want to make a script that when I write "User1" in the field "User_acc" from FormB the field "Note" of FormB to show all the "User_link" data from FormA assigned to that user, 

For example: I write in field "User_acc" from FormB: "User1"

and the script will show me in field "Note" from Form B: "Linnk1 | Link2 | Link3"

I am interested of the script part that does the fetch from FormA and write it in "Note" field in FormB.