You need to use %SMSRB for the body text of an SMS message. %SMSRF contains the sender information.
You may have to use regex to do the sentence match if it could contain capital letters one time and not the next time, or stray punctuation, ect. Minimally, I would surround it with *. For example, *your sentence here* An asterisk * matches zero or more characters.
You could also do this logic in the profile event. On the content line put your key words surrounded by *. Again, the trick is, you need to capture the possibility of capitalization or punctuation if others may send this text. Easier to use regex if you don't have control over the sending of the message
Then you can simply use an launch app action in the task.