You need to call AddtoCustomer with all the parameters not only single one.
By the signature above:
Public Function AddtoCustomer(FirstName As String, LastName As String, Address As String, Gender As String, Email As String, Password As String) As String
the function receives 6 parameters, returns string and should be called like this:
Dim rows as String = AddtoCustomer("Sinisa", "Hajnal", "My address 32", "M", "my-email@yes.no", "***")