User Name / repeats / backend / firebase

I have a marketplace app where users can upload their own post and I would like to add the following function

  • When the app opens, it asks for a create user name thatis storage in the backend so the idea is when a user wants to make a post, the post will be posted including the user name that he created.
  • I tried to add some functions in the posting page. I created a field call name it is repeating with the (backend). It is visible so the user knows the name that the post is gonna be showing but the problem is when the users clicks in post button it does post but not the name (I´m using create record) but in the custom object of the create record it doesnt allow me to set the in the user field
    Any idea how can I fix it?