In Teamleader Focus, there is a distinction between standard fields and custom fields that you create yourself. Whereas standard fields cannot be made mandatory, it is no problem for custom fields.

1. Go to "Settings" > "Custom fields"
2. Create or edit a custom field.
3. Click on "Advanced options"
4. Put the slider next to "Required field" on "Yes"
5. From now on the field needs to be filled in before the item, e.g. contact, invoice, etc. can be saved.