EFM - Can anyone help with the correct 'function' syntax

Hi

I am creating a bank payment file using the new EFM add-on in 8.81.

One of the fields I need to map is a calculation to work out a date which is 3 WORKING days from the payment date

EG. Payment date ~Thursday (7th July) will return Tuesday (12th July) as being 3 working days.

I realize that I need to use a fuction, but can not find an appropriate one, can any one help

Thanks very much in advance

Nicola