Can't get form to autopopulate

I need to populate a vendor form from fields in a client form. I have a dropdown field that I've populated with First Name and Last Name fields from the client form, but I'm not sure of the syntax for the, "on user input" function that will populate the rest of the vendor form. Here is what I have so far from your examples:

if (Voucher_Clients[First_Name Last_Name == input.Select_Client_Name].count() > 0)

{

temp = Voucher_Clients [First_Name Last_Name == input.Select_Client_Name];

Client_Number = temp.Client_Number;

First_Name = temp.First_Name;

Last_Name = temp.Last_Name;

}

Thanks in advance for any help!