Create a function for generating random number in SQL Server trigger

I have to create a function in a SQL Server trigger for generating random numbers after insert. I want to update the column with that generated random number please help what I have missed in my code.

If you know other ways please suggest a way to complete my task.

This my SQL Server trigger:

ALTER TRIGGER [dbo].[trgEnquiryMaster]
ON [dbo].[enquiry_master]
AFTER INSERT 
AS 
    declare @EnquiryId int;
    declare @ReferenceNo varchar(50);
    declare @GenReferenceNo NVARCHAR(MAX);

    select @EnquiryId = i.enquiry_id from inserted i;
    select @ReferenceNo = i.reference_no from inserted i;
BEGIN
     SET @GenReferenceNo = 'CREATE FUNCTION functionRandom (@Reference VARCHAR(MAX) )
        RETURNS VARCHAR(MAX)
        As
        Begin
        DECLARE @r varchar(8);
        SELECT @r = coalesce(@r, '') + n
        FROM (SELECT top 8 
        CHAR(number) n FROM
        master..spt_values
        WHERE type = P AND 
        (number between ascii(0) and ascii(9)
        or number between ascii(A) and ascii(Z)
        or number between ascii(a) and ascii(z))
        ORDER BY newid()) a

        RETURNS @r
        END
        '

        EXEC(@GenReferenceNo)

    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON

    -- update statements for trigger here
    UPDATE enquiry_master 
    SET reference_no ='updated' 
    WHERE enquiry_id = @EnquiryId
END   

via Chebli Mohamed