Join based on temp column

I have two tables that need to be joined, but the only similar column has excess data that needs to be stripped. I would just modify the tables, but I only have read access to them. So, I strip the unneeded text out of the table and add a temp column, but I cannot join to it. I get the error:

Invalid column name 'TempJoin'

SELECT
    CASE WHEN CHARINDEX('- ExtraText',a.Column1)>0 THEN LEFT(a.Column1, (CHARINDEX('- ExtraText', a.Column1))-1)
         WHEN CHARINDEX('- ExtraText',a.Column1)=0 THEN a.Column1
    END AS TempJoin
    ,a.Column1
    ,b.Column2
FROM Table1 as a 
LEFT JOIN Table2 as b WITH(NOLOCK) ON b.Column2=TempJoin