Select query to generate 8 digit alphanumeric key in SQL Server

i create a table tbl_channelpartner 

where cpcode is 8 digit 

my aim is when i generate guid 8 digit first it check that it not exists in cpcode if exist then create new one

purpose is uniqnes between cpcode it is primary key in my case 

i write this query but it not work 

SELECT SUBSTRING(CONVERT(VARCHAR(255), NEWID()),0,9) where NOT IN ( select cpcode from tbl_channelpartner)