(touser) is meant to be a parameter, as in:
This is a keyword and doesn’t have a parameter so it doesn’t work.
You want to use (sender):
keyword add sendto Hello there, (sender)
[12-03-2018 @ 19:59:25.930 MST] [MUTED] @IllusionaryBot, keyword "sendto" added!
[12-03-2018 @ 19:59:30.133 MST] illusionaryone: sendto
[12-03-2018 @ 19:59:30.137 MST] [MUTED] Hello there, IllusionaryOne
Be sure to review how the tags work, in particular read the difference between (sender) and (touser).