Error Message creating UDF using JOINS

Trying to create a User Defined Function that accepts start and end date, and returns number of sales by card type VISTA between the dates a user inputs. I am using a JOIN of two tables
and common column is the CreditCardID

CREATE FUNCTION NumberOfSales
                               ( @Startdate datetime, @EndDate datetime,@SalesCount INT )
                              RETURNS TABLE
                              AS
                            RETURN
                            (
                                    SELECT                  
                                    @SalesCount =            CardType, COUNT (*) AS CountOfSales
                                          FROM                        AdventureWorks2016.Sales.SalesOrderHeader a
                                          INNER JOIN                  AdventureWorks2016.Sales.CreditCard      b
                                          ON                              a.CreditCardID = b.CreditCardID
                                          WHERE                        CardType = 'Vista'
                                          AND                              b.ModifiedDate BETWEEN '2007/10/01' AND '2013/10/31'
                                    )
                              --Verify Function

                              SELECT * FROM NumberOfSales('2007/10/10','2013/10/19')
      
Msg 102, Level 15, State 1, Procedure NumberOfSales, Line 9 [Batch Start Line 135]
Incorrect syntax near '='.