Add script via GTM

1. Create DOM Ready trigger (if you already have this trigger, let's move to the next step)
2. Add Tag
3. Publish changes

Step 1: Click to create new trigger

Step 2: Enter trigger name and click on trigger configuration section

Step 3: Choose DOM Ready trigger

Step 4: Save the new trigger

Step 1: Click to create new tag

Step 2: Enter tag name and click on tag configuration section

Step 3: Choose tag type - Custom HTML

Step 3: Add the script to the HTML section and choose our created DOM ready trigger

Step 4: Save the tag

Step 1: Click to submit changes

Step 2: In this step we can see our changes and we can click to publish them

Step 3: Here we can enter Container version name and description and click Continue. Otherwise, if we don't want to enter anything here, we can just click to Skip

Now the script should be added to your website.