Learn more about Custom Fields here.
1. Click Agency Admin from the System Navigation Bar
2. Click the Agency Settings tab
3. Select Custom Fields from the list on the left
4. Select which Field Type you wish the new Custom Field to be (Radio, Checkbox, Textbox, Dropdown, Large Textbox) - See Know Your Field Type! here
5. Enter the Field Name
6. Click Save
7. Custom Field will be listed on the left under Existing Custom Fields