Google Tag Manager already on your site. If it isn't, it's simpler to paste the install code straight into your site: see the install guides.
Use it exactly as it is. Don't change the address or download the file: the chat updates itself from there.
You only paste it once. Colours, messages and everything else change from the dashboard and reach your site within about a minute.
Add the code as a tag
In Google Tag Manager, open your website's container and go to Tags › New.
Name the tag, for example "Live chat", and choose Tag Configuration › Custom HTML.
Paste your install code into the HTML box.
Under Triggering, choose All Pages. A DOM Ready trigger works too.
Save the tag, then Submit and Publish the container.
Before you publish, Preview shows you the tag firing on your site.
About the installation check
The dashboard's installation check reads your page's HTML, so it can't see code that Tag Manager adds after the page loads, and may say "Not found yet". That's fine: if the chat button shows on your site, it's working.
The chat on some pages only
Use a Page View trigger with conditions (for example, Page Path doesn’t contain /thank-you) instead of All Pages. Or keep All Pages and list the pages to skip under Chat Widget › Behavior › Pages without the chat.
If the chat doesn't show
The container isn't published. A saved tag does nothing until you Submit and Publish.
A consent tool holds the tag back. If your cookie banner blocks tags until a visitor agrees, the chat waits too. Decide with your advisor which consent group the chat belongs in.
An ad blocker blocks Tag Manager in your own browser. Try a private window without extensions.
Check that it works
Open your site in a private (incognito) window, so you see it the way a visitor does.
Look for the chat button in the bottom corner of the page, then send yourself a test message.
In the dashboard, the message appears in your Inbox as it arrives. Reply, and watch the answer reach the website.
You can also choose check the installation on the Install tab of Administration › Chat Widget: we load your home page and look for your code. No button? Go through when the chat doesn't show.
Set up the rest
The chat is on your website. A few minutes in the dashboard makes it yours: