If you're looking to send a message to subscribers based on the keyword they used to opt-in, you're in the right place.
Instructions
Select your list.
Click the New Message button.
Create your message as normal. Messages can be SMS or MMS.
Once you've finished composing your message, scroll down to Schedule.
Select one of the after opt-in choices. Add when you want the message to send.
Under Recipients, select Most Recent Opt In Keyword in the dropdown down, leave "Is" and then enter the keyword you wish to send the message.
It is important to make sure the keyword has been added to the list.
Select Save as Draft or Send depending on the needs for that message.
Additional Notes
The Opt-In keyword associated with a subscriber is set whenever a customer opts-in. If a customer opts-in, then uses an keyword in an attempt to opt-in again, the opt-in keyword will not be changed. If a customer opts-out, then opts-in again, this time with a new keyword, that new keyword will be set as that customer's most recent opt-in keyword.