Appending Custom Inputs to FormData using useActionState

[enzzzooo]

I am looking to use a custom date picker, instead of the ugly default html component.

I can use a callback function
<DatePicker onChange={handleDateChange} />
to add the selected date to the formData using .append(). Though it does not list this anywhere in the docs.
I am using useActionState so how does the function in
const [formState, formAction, isPending] = useActionState(createTrip, {})
recieve the new formData that includes the date?

In my server action I am looking to access the date just as if it was a normal input component like
const date = formData.get("date")