Teams Manager allows you to provision content from a number of apps in your Templates, and Planner is one of those apps.
Here's how you can ensure that both the app and the content within it are correctly provisioned:
Create a regular team in Microsoft Teams.
Add the Planner widget along with the content you want the end users to see.
Navigate to Settings > Templates in Teams Manager.
Click on Import from existing team.
Select the team that you've created and wait for the import process to finish.
Your new template will appear in the Active templates list.
Whenever a team is created with this Template, the system will automatically add both the Planner widget and the corresponding content to it.