You can go into a number of places to turn this on.
Once turned on, you will be charged the $5/month or $50/year add on fee.
Before you turn on this option, make sure it is right for your needs. Check out the info in this article: http://help.pastorsline.com/en/articles/6776320-v3-local-a2p-number-over-shared-numbers
NOTE: Turning on the PastorsLine shared short code is not the same as enabling short code keywords. For more information about short code keywords: http://help.pastorsline.com/en/articles/4223197-v3-free-vs-paid-short-code-keywords-scks
OPTION 1
Admin Settings --> Organization Profile --> Toggle on the Enable Short Code button (blue box in image below). The button will turn purple.
OPTION 2
App Settings --> General --> Messages --> Toggle on the Enable Short Code button (blue box in image below). The button will turn purple.
OPTION 3
Admin Settings --> Numbers --> Click on the 3-dot Menu (yellow arrow in image below)
--> Click on Release Short Code button (blue box in image below).