1: Click Admin.

2: Select General Settings from the drop down list - the General Settings window will open.

1: Click the Invoices tab - the Invoices window will open.

2: To make the "Their Reference" Compulsory/Mandatory click the check box. To turn this off, untick the check box.

3: Click the OK button to close the window and Save the changes.

When the tick is on - the "Their Ref" field is highlighted in red to show that is a Mandatory field.