There are two options:
1.) You can simply click the Log in link, and in the Google authentication page that opens, click the upper right corner to add the other account. However, in some cases this may lead to you being logged out of the account you use with Google Sheets/Google Data Studio.
2.) A better way is to follow these steps:
- Right-click the "Log in link", copy the URL
- Left-click the "Log in" link
- For logging in to the other Google Account, open another web browser, or an incognito window in the browser you're using
- Paste the link into the new browser window you opened, and log in with the Google Account you want to use with the data source (eg. Google Analytics, Google Ads etc.)