You'll need to have the Account to Customer mapping correctly configured as well as the Project to Project mapping.
In the Project to Project mapping, scroll down to section 4 'Create Clarizen Links' and configure it like so:
Then, verify in your Account to Customer mapping that you key fields are using a unique identifier, so not a text field like 'Name'.
Here's an example of a properly configured Account to Customer mapping using a unique key field:
Please note that every organizations configurations can be highly customized, so if this doesn't work for you, please reach out to email@example.com to investigate the issue. This will not create links retroactively, so only new project records going forward. If it's a custom integration, we may need to involve professional services to insure a successful solution.