Follow these steps to "Harness your Website" on your Drupal 8:

Step 1: Log in to your Harness Admin Dashboard

Step 2: In your "Settings" - click on the "Website Code" tab (or Click Here)

Step 3: Copy the "Harness your Website" Code

Step 4: Log in to your Drupal 8 Admin Account

Step 5: In the upper menu, click on "Structure"

Step 6: Click on "Block layout" and then select "Custom block Library"

Step 7: Click the button "Add custom block"

Step 8: Add a title (You can put "Harness Code")

Step 9: Select "Full HTML" on the dropdown

Step 10: Click on "Source" and then paste your Harness Code - then click "Save"

Step 11: Click on the "Block layout" tab

Step 12: Find the footer in the list and click on "Place block"

Step 13: Look for the block you just created in the list and click "Place block"

Step 14: Uncheck "Display title" and click the "Save block"

Step 15: All done! Remember to check your website setup with Chrome or Firefox