Hi Fa,
It depends on the type of your custom fields.
For example, here is a custom field called num in my Contact entity and its type is Single Line of Text.
When creating the Form Field, select as the following image and it will be shown in the end of the drop list.
If you select the type of currency, it will only show the fields whose Data Type is currency.
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya
