You need to enter client ID and client secret into license dialog to use Query Viewer.
License key is tied to client ID. Hash of client ID is embedded into license key. Thus, license key is only valid for specific client ID. And client secret is not tied to license key.
Client ID and client secret are required to access Search Console data of sites which Web Masters manage through Google Search Console API. For procedure to obtain them which is very lengthy, please see this page.
To try Query Viewer, use trial license. Click "Register Trial License Key" button to create and register trial license key with client ID embedded.
Dialog looks like this.
Now you can try Query Viewer. It never expires.
In case you need to change client ID, click "Unregister License Key" button. You can type in client ID again.
Closing dialog makes app start accessing Google Search Console data. You may be asked to login Google account and then agree to give access right to Query Viewer.
You may see error screen that tells you it could not establish access to somewhere. But please simply ignore that.
For license management, please see this page.