Need help in filling US custom declaration form when surname is blank on passport

[anonymous89]

Hi,

My parents are travelling to USA next month.
On their passport the SURNAME filed is BLANK and their full name is provided in GIVEN NAME field.
They got US visitor visa with SURNAME= 'Given Name of passport' and FIRST NAME= FNU.

I am looking for answers on how they have to fill custom declaration form. The declaration form has following fields.

1. FAMILY NAME
2. FIRST (GIVEN)
3. MIDDLE

Would appreciate if someone can answer here.
Should they be filling in details from VISA STAMP ? i.e FIRST (GIVEN) = FNU and FAMILY NAME: 'full name as in passport's GIVEN NAME section' ?
Or should they be filling in only passport details ?

[immihelp]

You can either go by the passport or by the visa. Either way would work.

However, when buying travel insurance (available at https://www.insubuy.com), go by the passport.