Using Google optimize snippet with Google Analytics +

I'm trying to add Google optimize snippet into existing Analytics tracking code - https://support.google.com/optimize/answer/6262084
Looking into Google Analytics + plugin but can't find any option where that can be added.

The Analytics code seems to be inserted in page head tag automatically by the plugin, what I want to do is inserting the additional snippet required there for Google Optimize using GA+.
ga('require', 'GTM-XXXXXX');