How do I pass parameters to function dynamically and execute stored procedure dynamically with EXEC()

ALTER PROC [dbo].[spGetanswer]
(
    @a            VARCHAR(10) = NULL,  
    @b            NVARCHAR(200) = NULL,
    @c            VARCHAR(15) = NULL,
    @d            NVARCHAR(MAX)= NULL,
    @StartDate    datetime = NULL,
    @EndDate      datetime = NULL,
    @col1         bit = 0,
    @col2         bit = 0,
    @col3         bit = 0,
    @col4         bit = 0,
    @col5         bit = 0
)
AS
BEGIN
DECLARE @statement nvarchar(max), @params nvarchar(max);
DECLARE @SqlSelect nvarchar(max), @SqlFrom nvarchar(max), @SqlWhere nvarchar(max), @SqlGroup nvarchar(max), @SqlOrder nvarchar(max);
    
    SET NOCOUNT ON;
    
    SET @SqlSelect = N'SELECT Sum(IsNull(SALES, 0)) As Sales, IsNull(Customer, '''') As Customer';
    SET @SqlFrom = N' FROM dbo.VW_getanswer ';
    SET @SqlGroup = N' GROUP BY Customer ';
    SET @SqlOrder = N' ORDER BY Sales DESC, Customer ';
    
    
    SET @SqlWhere = N'';
    
    If @StartDate Is Not Null And @EndDate Is Not Null
    BEGIN
        SET @SqlWhere = @SqlWhere + N'And Convert(datetime, [MONTH]) Between @StartDate And @EndDate ';
    END
    Else If @StartDate Is Not Null
    BEGIN
        SET @SqlWhere = @SqlWhere + N'And Convert(datetime, [MONTH]) >= @StartDate ';
    END
    Else If @EndDate Is Not Null
    BEGIN
        SET @SqlWhere = @SqlWhere + N'And Convert(datetime, [MONTH]) <= @EndDate ';
    END;
    
    If @Company Is Not Null
    BEGIN
        SET @SqlWhere = @SqlWhere + N'And Company = @Comany ';
    END;
    
    If @a Is Not Null
    BEGIN
        SET @SqlWhere = @SqlWhere + N'And a In (SELECT Item FROM dbo.SplitString(@a, '','')) ';
    END;
    If @b Is Not Null
    BEGIN
        SET @SqlWhere = @SqlWhere + N'And b In (SELECT Item FROM dbo.SplitString(@b, '','')) ';
    END;
    If @c Is Not Null
    BEGIN
        SET @SqlWhere = @SqlWhere + N'And c In (SELECT Item FROM dbo.SplitString(@c, '','')) ';
    END;
    If @d Is Not Null
    BEGIN
        SET @SqlWhere = @SqlWhere + N'And d In (SELECT Item FROM dbo.SplitString(@d, '','')) ';
    END;
    
    If @SqlWhere != N''
    BEGIN
        -- If we have a filter, replace the first "And" with "Where":
        SET @SqlWhere = STUFF(@SqlWhere, 1, 3, N' WHERE');
    END;
    
    
    If @col1 = 1
    BEGIN
        SET @SqlSelect = @SqlSelect + N', col1';
        SET @SqlGroup = @SqlGroup + N', col1';
        SET @SqlOrder = @SqlOrder + N', col1';
    END;
    If @col2 = 1
    BEGIN
        SET @SqlSelect = @SqlSelect + N', col2';
        SET @SqlGroup = @SqlGroup + N', col2';
        SET @SqlOrder = @SqlOrder + N', col2';
    END;
    If @col3 = 1
    BEGIN
        SET @SqlSelect = @SqlSelect + N', col3';
        SET @SqlGroup = @SqlGroup + N', col3';
        SET @SqlOrder = @SqlOrder + N', col3';
    END;
    If @col4 = 1
    BEGIN
        SET @SqlSelect = @SqlSelect + N', col4';
        SET @SqlGroup = @SqlGroup + N', col4';
        SET @SqlOrder = @SqlOrder + N', col4';
    END;
    If @col5 = 1
    BEGIN
        SET @SqlSelect = @SqlSelect + N', col5';
        SET @SqlGroup = @SqlGroup + N', col5';
        SET @SqlOrder = @SqlOrder + N', col5';
    END;
    
    SET @statement = @SqlSelect + @SqlFrom + @SqlWhere + @SqlGroup + @SqlOrder;
    SET @params = N'@a VARCHAR(10), @b NVARCHAR(200), @c VARCHAR(15), @d NVARCHAR(MAX), @StartDate datetime, @EndDate datetime';
    EXEC sp_executesql @statement, @params, @a = @a, @b = @b, @c = @c, @d = @d, @StartDate = @StartDate, @EndDate = @EndDate;
END;