Pre-requisites:
'Admin' permission on the project template where a system task needs to be created.
Steps to create a system task for executing a Groovy script on custom objects in a project template are as follows:
Follow the steps to create a system task for custom objects in a project template (steps 1 to 4).
Next, follow the steps to create a system task to execute a Groovy script on custom objects in a project (steps 2 to 3).
Once done, click the 'Create' button to create a system task.