How to use cURL to send Cookies?

You are using a wrong format in your cookie file. As curl documentation states, it uses an old Netscape cookie file format, which is different from the format used by web browsers. If you need to create a curl cookie file manually, this post should help you. In your example the file should contain following line

127.0.0.1   FALSE   /   FALSE   0   USER_TOKEN  in

having 7 TAB-separated fields meaning domain, tailmatch, path, secure, expires, name, value.

This worked for me:

curl -v --cookie "USER_TOKEN=Yes" http://127.0.0.1:5000/

I could see the value in backend using

print(request.cookies)

You can refer to https://curl.haxx.se/docs/http-cookies.html for a complete tutorial of how to work with cookies. You can use

curl -c /path/to/cookiefile http://yourhost/

to write to a cookie file and start engine and to use cookie you can use

curl -b /path/to/cookiefile  http://yourhost/

to read cookies from and start the cookie engine, or if it isn't a file it will pass on the given string.