First, follow this guide - https://docs.aws.amazon.com/codecommit/latest/userguide/setting-up-ssh-unixes.html#setting-up-ssh-unixes-keys and make sure you've set up SSH access to your CodeCommit repository.
When you create your template in env0 - you will need to create a template using the "Other VCS" type.
For the URL - you'll need your CodeCommit user name, it will look like this - (using the values from the tutorial above) -
ssh://APKAEIBAERJR2EXAMPLE@git-codecommit.us-east-2.amazonaws.com/v1/repos/my-repo-name
To complete the setup - you will have a place to set the SSH key for the template, where you'll need to copy and paste your private SSH key for the repo (created in the tutorial above as a local file).