Within Netbookings settings it is possible to enable which fields are mandatory for guest creation in front end and back end, for waivers and voucher creation.

Includes the following fields (Applicable to Guest, Extra Guest, Voucher and Waiver): Car Registration , Country, E-mail, First Name, Last Name, Number and Street name, Phone number, Post code, State and Suburb.

Modify Mandatory Guest Fields

Go to System>Setup>Settings and navigate to General tab. 

Here we can see the ‘Mandatory guest fields’ panel.

In this example we add Post Code as a mandatory field for Guest (as well as Voucher and Waiver) and click ‘Save’.

Now on front end when guest creates profile they are required to provide a post code as indicated by red asterisk.

Same would apply when creating voucher or signing a waiver as enabled above.