Thanks for your reply, but I figured it out.
Looks like the main issue was that the call needs to get a normal string, but is also expecting "Content-Type: application/json". Took me a lot of trial and error to get it right, but it works now.
curl -X POST
--header "x-qlik-xrfkey: 0123456789abcdef"
--header "X-Qlik-User: UserDirectory=internal;UserId=sa_repository"
--header "Content-Type: application/json"