If the customer provides you with an access, you will receive an email. Click on "Create Staff Account" and follow the instructions.
When you logged in to Shopify Store, you need to go to "Themes" (under Online Store). Click on "Actions" (right from Live theme) & Edit code.
If you don’t see "Themes" or cannot click on it, please contact the shop owner and ask to provide additional privileges so you can customize live themes.
Open “theme.liquid” in the “Layout” folder, and paste the Mgid pixel at the end of the head tag in “theme.liquid”, after “” and before the closing “</head>” tag. Don’t forget to click the “Save” button in the top-right corner!
Go to the site and confirm that the code is there.