The post-install guide tells users to send a POST request to the server before setting up a reverse proxy to create a user. However the page doesn't mention setting the headers that .NET expects. It could be helpful to provide an example command like:

curl -X 'POST' \
  'http://localhost:3000/api/iceshrimp/auth/register' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/json' \
  -H 'Host: <webdomain>' \
  -d '{
  "username": "",
  "password": ""
  }'