On the Channels page you need to scroll down to the bottom where you will see the title ‘API’. To the far right of this you will see a button called ‘Add API client...’ Press this and fill out the details.

After this you will see the client name appear under ‘API’ and to the far right of this is the option ‘Edit client...’
You will be taken to the edit client screen, here you need to make sure that the box under ‘Trusted’ is ticked.
 

Press update and you will be taken back to the API tab, under the client name you will find the API key that you will need to enter to create your widget.
 
At the moment the UserVoice widget will not work if your Forum is set to Private, it is something we are looking into but for now the widget will show nothing in all results.