We couldn't authenticate with the credentials provided.

Hi js15 ,
Thanks for confirming that the API works in the browser and likely in Postman too , that is really helpful.
The issue might be with how the API key is being passed. Power BI Web API authentication screen does not always handle API keys the way some services expect, especially if a specific header name like Authorization is required. Instead of using that screen, try using the Web connector with the Advanced option. This lets you manually enter the full API URL and add a custom header. For example, if the WTO API expects the key as a Bearer token, you may add a header like Authorization with the value Bearer YOUR_API_KEY, or just the key if that is what the documentation says. This approach gives you more control over the request and usually works better for APIs that are not tied to a username/password or organizational login.

Refer the document here for more information:
https://learn.microsoft.com/en-us/power-query/connector-authentication 
https://learn.microsoft.com/en-us/power-query/connectors/web/web-troubleshoot 
https://learn.microsoft.com/en-us/power-query/handling-authentication 

If this solution resolve your query,consider accepting it as solution.

Regards,
Pallavi.