To start running A/B tests on your website, all you have to do is adding a snippet/tag of JS code to your website.
This JS snippet contains the unique ID of your project.
It automatically updates when you change relevant data on the plattform. Once
installed you don't have to change it again. 

This is how your snippet looks like (here with Project ID 12345678):

<script src="https://cdn.ablyft.io/s/12345678.js"></script

How to implement the Snippet via Google Tag Manager?

The snippet (see above) needs to be placed in a new Tag in Google Tag Manager.
Example:

Where to find the Snippet?

You find  the snippet under "Project > Snippet".

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