The projects API endpoint response contains the runners_token key. You can use this to automatically fetch the runner tokens for any project.

You can then use that in a few ways. One way would be to have your runner registration script fetch the runner token itself such as with this example:

curl --fail --silent --header "Private-Token: ${GITLAB_API_TOKEN}" "https://$GITLAB_URL/api/v4/projects/${PROJECT}"

Or you could use the Gitlab Terraform provider's gitlab_project data source to fetch this from whatever is running Terraform and then inject it into the thing that runs the registration script such as a templated file:

data "gitlab_project" "example" {
  id = 30
}

locals {
  runner_config = {
    runner_token = data.gitlab_project.example.runners_token
  }
}

output "example" {
  value = templatefile("${path.module}/register-runners.sh.tpl", local.runner_config)
}

For a specific GitLab runner tied to a project, the token can be found in Step 3; in Project -> Settings -> CI/CD -> Runners in the Setup a specific Runner manually section.

You will also be able to find the URL which you need to use to register the runner in Step 2.