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Create a new GTM account at http://tagmanager.google.com , or use an existing account that you’ve already setup for your business
Login to your GTM account and copy your Google Tag Manager Container ID
Login to VIA
In the left hand menu, go to Settings > Configuration
Go to Third Party > Google > Tag Manager
Paste your Google Tag Manager Container ID into the Code field
You have now added linked your GTM account to your website. The information will automatically be saved once it has been entered so you can close this tab
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