ON is F12 in tilem as well iirc. I can't test this right now though as I don't have an F12 on my desktop's keyboard.

Ah ? Weird, I pressed F12 to turn the calc on but it didn't work
Or maybe it was not F12 that I try even though I thought it was -.-

Also, here's what I found on the PDF on TilEm's website:
"The keybindings are defined in a keybindings.ini file (usually copied into /.config/tilem2/keybindings.ini).
There's currently no tool to interactively edit this file, but you can edit it by hand if you do this carefully"