Creating custom fields
From Planfix
Creating custom fields is done in the Account management section under Custom fields > Task fields:
Using the button New field:
When creating a field, you can select which set to create it in:
Once you have created the field, you can add it to the required objects, where it will appear as follows: After a field has been created, it will be displayed in the corresponding field set in task templates. In order for a field to appear in a task template, you must move it from its current field set to the desired task panel:
Only after this is done will the field appear in the template and be available for use in tasks created using the template: