If you have no set any start- or end date and time when creating the AB test code in the form above, you can use GTM's scheduling for tags.
4. Preview & Publish container
Preview your test before publishing your new GTM container and switch variations using the Lean CRO preview panel. Note, if you're using GTM scheduling, your tag will not fire until the scheduled date/time, even in GTM preview mode.
Go into Google Tag Manager Preview mode.
Browse to your test url with this url parameter: http://www.yourdomain.com/your-test-page?preview_id=2
Publish your Google Tag Manager container and your test will go live on your scheduled date. Hooray!