/data modify storage ... CUSTOMNAME

I'm making my first datapack ever to utilise on a small server with some friends this summer. I want to create a list which every member can append to via a /trigger.
I've recently found out about /data modify, and I think it's the best I can use.
Currently I get user input via a specifically named book_and_quill. I might change this in the future, but i'm able to run a command to get the written text in there. 
Where i am struggling, is to get that custom string as a key-name in the storage. 
for example if i wrote "datapack", it should append t the storage with a key being "datapack" and the value being my current coordinates.
I'm not that good at explaining myself, so sorry if this is a hassle to read.
TLDR: I want a key in storage with user input caused by /trigger.
feel free to ask if you need me to elaborate