So you've finalized your web widget and are ready to assign it to a campaign? 
To do that, just edit the campaign that you want to use it with, then go to Step 2. Then find the Subscription by Widget option and enable it.

Next, go back to the Web Widget section, and you'll find a yellow icon with "</>" that indicates a script within the other options in the Actions menu of the widget. 
That icon/option contains the Embed Code of the Widget. Click on that. then select the campaign that you want to generate the code for, then click on Get Embed Code.

Grab the code then you can place it as an HTML script on your webpage. 
Note: Make sure to place it on the body, not on the header/footer.