To add a custom or default field to your add deal, contact, or product modal, go to Company settings > Data fields, and select the tab of the prompt you want to add to add the field to. You can then click on the data field (custom or default) you want to add to your add item prompt to update its "Appears in... view" properties. 

For example, if you want to add a person custom field to the add deal or add person modal, you can go to the Person tab and click on the custom field you want to include. For people fields, you will have the option to add it to the Add deal or Add person modal.  

Once the option has been selected, it will now show up in the Add deal or Add Person modal. 

Which fields can I add?

The ability to add a data field to an add item modal will depend on what kind of field you are trying to add, and where you are trying to add it.