Schedule a Job to Start Automatically when SQL Server Agent starts

SQL Server Agent jobs can be set to start automatically when SQL Server Agent starts.
To schedule a job to start automatically when SQL Server Agent starts, expand SQL Server Agent in SQL Server Management Studio Object Explorer, right click 'Jobs' and choose 'New Job'

job_new

Select 'Schedules' tab and click 'New...':

job_new_schedule_automatic_start

Give a name to the schedule, and choose 'Start Automatically when SQL Server Agent starts' option from the 'Schedule type' drop down list:

job_schedule_automatic_start

Click 'OK', and the schedule to start job automatically when SQL Server Agent starts is set:

job_schedule_automatic_start_set

To see how to list all jobs set to start automatically when SQL Server Agent starts, read related blog post:
List all Jobs set to Start Automatically when SQL Server Agent starts