I have encountered this and could not find a provision within George.
I have adapted the following which can be amended as your job No changes with every job.
Go to System
Then Job Types.
In Job Types create an entry within this to say " commercial window cleaning. JOB No. 1234"
Then select your particular customer and change the job type to your newly created specific Job No 1234 via amending details of a customer.
Don't forget to reinstate the original job type rather than the new one.
Of course you could simply enter the Job No by hand ! !