'Random Targeting' for Phishing Emails will be set up via the area Target. See article: How to Create and Publish a Phishing Simulation - Step 6 (Target)

• If you wish to use Random Targeting, tick the Random Targeting checkbox:

• You will then be shown the below notification:

• Select Confirm.
• Next, select the percentage of users you would like to be targeted with the Phishing Emails:

• And finally, select your targeted users.