Select the campaign you wish to create a keyword for by clicking the Manage button from your list of campaigns on the campaign dashboard.
Click the Subscribers icon in the campaign navigation bar in the top right hand corner of your campaign homescreen.
Click the green Add Subscriber button in the right hand corner.
Enter your desired keyword in the field below "Keyword".
Click the Save Changes button at the bottom of the screen.
If you're looking to create multiple keywords for one campaign, click here.