Please Note: These instructions were provided to us by a client who has successfully integrated Google Optimize in the manner that follows. TrueCommerce/Nexternal has not tested this, and we cannot consult if you are unsuccessful.
In order to integrate Nexternal with Google Optimize when you already have Google Analytics integrated with data in the Google Analytics Account and Google Analytics Domain fields, leave that integration intact (and verify that it is working), and then add a second Google Analytics integration with the Google Optimize snippet in the Tracking Code section. (Settings / Edit Policies Addenda and Disclaimers / Tracking Code):
<!-- Google Analytics w Optimize snippet -->
ga('create', 'UA-111111111-1', 'auto');
<!-- END Google Analytics w Optimize snippet -->
In order to test, use Google Tag Assistant extension. There will be two Google Analytics integrations when tested but it should be fine.