How to generate random number using character between A-F and 0-9 in SQL Server

CREATE procedure dbo.GenerateRandomString (
 @useNumbers bit,
 @useUpperCase bit,
 @string varchar(100) OUT
)
As
Begin
	DECLARE @charactersToUse VARCHAR(8), @stringLength INT
	SET @charactersToUse='ABCDEF'
	SET @stringLength=8
if @stringLength <= 0
 raiserror('Cannot generate a random string of zero length.',16,1)

declare @characters varchar(100)
declare @count int

set @characters = ''

if @useNumbers = 1
begin
 -- load up numbers 0 – 9
 set @count = 48
 while @count <=57
 begin
     set @characters = @characters + Cast(CHAR(@count) as char(1))
     set @count = @count + 1
 end
end

if @useUpperCase = 1
begin
 -- load up lowercase letters a – z
 set @count = 97
 while @count <=122
 begin
     set @characters = @characters + Cast(CHAR(@count) as char(1))
     set @count = @count + 1
 end
end

set @count = 0
set @string = ''

-- If you specify a character set to use, the bit flags get ignored.
if Len(@charactersToUse) > 0
begin
 while charindex(@charactersToUse,' ') > 0
 begin
  set @charactersToUse = replace(@charactersToUse,' ','')
 end

 if Len(@charactersToUse) = 0
  raiserror('Cannot use an empty character set.',16,1)

 while @count <= @stringLength
 begin
    
set @string = @string +
SUBSTRING(@charactersToUse,CAST(ABS(CHECKSUM(NEWID()))*RAND(@count) as
int)%LEN(@charactersToUse)+1,1)
     set @count = @count + 1
 end
end
else
begin
 while @count <= @stringLength
 begin
    
set @string = @string +
SUBSTRING(@characters,CAST(ABS(CHECKSUM(NEWID()))*RAND(@count) as
int)%LEN(@characters)+1,1)
     set @count = @count + 1
 end
end

end
GO