Mailgun API authentication issues - getting 401 error

I’m trying to use Mailgun to send emails through Postman but keep running into authentication problems. No matter how I format my API key in the authorization header, I always get a 401 unauthorized response.

I’ve attempted different ways of setting up the auth in Postman but nothing works. Even when I try using curl like the docs show, I get a forbidden error:

curl -s --user 'api:xyz123...' \
    https://api.mailgun.net/v3/sandbox123.mailgun.org/messages \
    -F from='[email protected]' \
    -F to='[email protected]' \
    -F subject='Test Message' \
    -F text='This is a test email message'
Forbidden

What’s the correct way to authenticate with Mailgun API? Am I missing something in the authorization setup?