Click on the dropdown menu in the top left to get a list of available projects with their names and IDs.
Use the ID of the project you want to use fill the template gcr.io/<PROJECT_ID> and get your URI (You can also use the other prefixes <us/eu/asia>.gcr.io as explained above if you want your images stored in a different region).
When using the Google Artifact Registry, the GCP container registry URI should have the following format:
Go here and select the repository that you want to uses to store Docker images. If you don't have a repository yet, take a look at the deployment section.
On the top, click the copy button to copy the full repository URL.