Login to your Shopify Admin
From the left menu, go to Online Store ->Themes
Click Actions -> Edit code for the Live theme.
Click on theme.liquid in the Layout section.
Locate the <head> section of the html template.
Copy/paste the value from the Omnivore settings.
In the screenshot below, we inserted the Google site verification tag immediately after the viewport meta tag, but it can be placed anywhere between <head> and </head>.
Click Save in the upper right corner.
Optional: How to check if the verification code is present on your website
Verify your website in Omnivore
Return to Omnivore and hit the Check Now button in Step 4 from the Set up now link on the Dashboard