Required custom fields
From Planfix
Task, contact, and project fields can be set to Required. This can be done when editing the object:
We find the desired field in the left-hand panel and open its settings by clicking the three dots. Then we click the checkbox next to Required:
When creating or editing a task, project, or contact, required fields will be marked with a red asterisk:
You won't be able to save the task, project, or contact without entering a value for these fields.
Required system fields
As of April 2019, system fields in tasks, projects, and contacts can also be required fields. The steps for making a system field required are the same as for a custom field.