How can I add custom fields in the "Manage Sales Task Templates" tool?

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We are currently trying to configure the automatic creation of activities (tasks), which is exhibiting unexpected behavior. This is due to a specific business requirement where we have added a custom field that is mandatory(*) on the landing page of the activities module. This is necessary because the activities object is shared by the sales and service objects, and we have configured a dropdown list with the goal of distinguishing which object the record belongs to.

During the conducted tests, we have encountered an error that prevents the automatic creation of the record, and this occurs because the custom field is mandatory for creating the activity. Therefore, we need to know if there is any way to add additional fields in the "Manage Sales Task Templates" task that allows for assigning custom fields.