How to POST the refresh token to Flask JWT Extended?
Try
curl -H "Authorization: Bearer $REFRESH" -X POST http://localhost:5000/refresh
For me only this worked, both were required - post data and Auth header.
curl -X POST -H "Content-Type: Application/json" -H "Authorization: Bearer $REFRESH" -d "{\"refresh_token\":\"$REFRESH\"}" http://localhost:5000/refresh