A custom policy definition is a way to define your own rules for using Azure resources. You can use custom policies to enforce compliance, security, cost management, or organization-specific requirements. However, a custom policy definition alone is not enough to meet the goal of automatically blocking TCP port 8080 between the virtual networks. You also need to create a policy assignment that applies the custom policy definition to the scope of the subscription. A policy assignment is the link between a policy definition and an Azure resource. Without a policy assignment, the custom policy definition will not take effect. Therefore, the solution does not meet the goal.
Reference:
Tutorial: Create a custom policy definition
Create and manage policies to enforce compliance