I am not able to reply to my existing query in the forum. Hence created new thread.
I did tried that injecting script in header.php, but it is not getting injected. Even I tried Header and Footer plugin.
Oh, for that. It should work by adding code to header.php just above wp_head();
Also you can use some plugin like Insert Headers and Footers.
<script src="//load.sumome.com/" data-sumo-site-id="c5eac674a593cc813fd70bc5220724f2132e79e954050471d5e825aa20581be2" async="async"></script>