GTAG for Optimize ID - google-analytics

Question

What is the gtag.js equivalent of the following analytics.js Google Optimize integration:
ga('set', 'expId', '$experimentId'); // The id of the experiment the user has been exposed to.
ga('set', 'expVar', '$chosenVariation'); // The index of the variation shown to the user.
I could not find anything documented in Google site.
Some places I looked:
https://developers.google.com/analytics/devguides/collection/gtagjs/migration
https://developers.google.com/analytics/devguides/collection/analyticsjs/experiments
Update - 08/23/2018 - using the response from #AnkDasCo
Further, my Optimize experiment is configured as SERVER_SIDE which means I do not want Optimize to assign the experiment and variant to the site. Instead, that's something I would like to control from my end. I have injected the following in the page:
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
gtag('set', {'expId': 'xxxxxxxxxxxxxxxxxxxxxx'});
gtag('set', {'expVar':'1'}); // set to variant id 1
gtag('config', 'UA-xxxxxxxx-y', { 'optimize_id': 'GTM-xxxxxxx'});
However, the variant assignment is still being controlled by Optimize and not by the script above. In the above example I am asking it to be set to variant id 1, but it's not happening.
Even setting the experiment/variant in a single statement as this does not work either:
gtag('set', {'exp': 'xxxxxxxxxxxxxxxxxxxxxx.1'});
Any thought what I might be missing. Any help will be greatly appreciated.

Answer 1

You can still set values that persist across gtag calls on the page.
gtag('set', {'expId': 'xxxxxxxxxxxxxxxxxxxxxx'});
gtag('set', {'expVar':'1'});

Answer 2

This analytics.js way works:
<script>
(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','https://www.google-analytics.com/analytics.js','ga');
ga('create', 'UA-xxxxxxxx-y', 'auto');
ga('set', 'exp', 'xxxxxxxxxxxxxxxxx.1');
ga('require', 'GTM-xxxxxx');
ga('send', 'pageview');
</script>
Note that since the experiment is set as SERVER_SIDE, the effect of setting the experiment and the variant needs to be handled by the server and Optimize plays no role in that. If you do server side, you do it all yourself - you can create an experiment for "page A" and then send experiment hits from "page B" and things will still work fine.