Go in to "OS Deployment, and right click your OSD point and click on "Configuration"
Click on "OSD Gui" and scroll down to the CustomFields"
Here, you have five opportunities, you can choose to your Custom Field.
There is a label shown in you OSDClient, and an default value.
WARNING: Default Value can only be TRUE or FALSE
Now pxe-boot your client, and you will have shown you Custom Fields as shown here: