Instructions
Select the list you wish to create a keyword for by clicking the Manage button from your list on the dashboard.
Click "Settings" and select in the dropdown "Mobile Opt-in".
Click the "+" button to add a keyword.
Enter your desired keyword in the new field (type over IMPERSONATE).
Click the Save Changes button at the bottom of the screen.
Additional Notes
If you're looking to create multiple keywords for one list, click here.