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