After you have created your survey and launched it as a Web Widget, you will find the script needed for installation under the Touchpoints tab on the navigation bar.

If you are using Google Tag Manager, you can also add the script directly there.
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Go to the Web Widgets tab and click Get website script and the script will open in a popup window.  

Add the script to the scripts-section of your site (typically at the end of the body tag or in the footer, varies a bit on what platform you use for example Wordpress etc.). The script is the same for all your website plugin touchpoints. 

Note: You can add different domains for your widgets when setting up the triggering parameters if you are using surveys on different domains. If you have more than one domain then name them accordingly, and when getting the script make sure you get the right script for the right domain.