Did you read documentation? Clearly said it need to be set as username.
The issue with this basic auth is that there’s no password. And Bubble, when you user basic auth, will automatically process to the base64 encoding of username:password and set an header Authorization: Basic base64encodedstring

You have two options to solve that. use the url to set the basic auth like
https://username:password@url.com or suggested alternatives methods like api_key in url or header (like documentation said for this API)