You still need to call
Button:RegisterForClicks() for the
OnClick handler to work.
It's also good practice to make sure
Frame:EnableMouse() or
Frame:SetMouseClickEnabled() is set.
Frame:EnableMouse() is a legacy function that controls both clicks and mouseover while
Frame:SetMouseClickEnabled() and
Frame:SetMouseMotionEnabled() individually control each of their respective features.
On a side note, you don't need a
SecureActionButton to send
/sit. It's processed as an emote using the
"SIT" and
DoEmote() is not protected. You can run it as
DoEmote("SIT").