On an auto number generation, I need to create one where it is a concatenation of a job number field and the auto number

For example:

If a user enters 122019 in the [JobNumber] then the auto number would generate as "1001". So the after the concate it would be "122019-1001"

Then another user comes along and enters 122019. As this is the second time this job number was entered it the auto number would generate as "1002". So after the concat it would be "122019-1002".

Then a third user comes along and enters 122011. Since this is the first time this job number is entered the auto number would generate as "1001". Which after the cancat it would be "122011-1001'