You can get an API key here: https://docs.mendix.com/developerportal/community-tools/mendix-profile/#profile-api-keys
You do need to use the mendix-apikey when calling the API.
I hope this helps.
Did you put the username and the API key in your headers like this?
And if screenshot in your question is your API key I would create a new one because it is now in the open. You should keep your API key a secret just like your passwords.