In the srm booking manager, under the config page, you have a properties sub-tab.
Here you can define one or more properties for your bookings.
You can also define if you wish to copy the property to your SRM service object.
If you use the default booking wizard, the users will automatically be presented with input fields for every property.
If you use a tailored booking wizard, an extension to the wizard will have to be made to provide a UI for the users to enter a value for the defined properties.