sharepoint online – allow the current user to programmatically assign a list item

I'm creating an event webpart associated with a list of items. When a user participates in an event, a JSON object is parsed into a TXT document attached to that particular list item / event. with the user details. However, if a user does not have edit permission, it will not work. In that case, I want to give the user edit permissions. I use sharepoint online.

I thought about using the REST API to change the user permissions for that particular list item, but as far as I can grasp it, it seems that it only works for group permissions, and not the current user. Did I miss something or do I have to use a different method?