How to set cron job which runs every half an hour?

After logging in, click on “Add new cron job”, enter a cron job name (optional), choose following configurations in time setting dropdown boxes:
Minute: */30
Hour: *
Day: *
Month: *
Weekday: *

Enter the URL you want to call and choose the rest settings if necessary, click on “Create” button, wait until the notice popups, you're done.
The final time pattern will be */30 * * * * after the cron job is added.