https://www.encorebusiness.com/blog/dynamics-crm-auto-number-entities/
https://community.dynamics.com/crm/b/razdynamicscrmconsultant/archive/2016/01/17/free-autonumber-solution-for-microsoft-dynamics-crm
https://community.dynamics.com/enterprise/b/dynamics365fordevelopers/archive/2017/10/02/native-dynamics-365-v9-auto-number-solution-for-custom-entities
http://celedonpartners.com/blog/celedon-autonumber-solution-for-dynamics-crm/
Working version: https://crm2015autonumber.codeplex.com/