you probably want the key object in combination with select. For more serious mapping it might be worth looking into coll rather than select… Have a look below and at the helpfiles of those objects. that should get you started.
And for key "combos" have a look at an object called thresh (there is also quickthresh).
– Pasted Max Patch, click to expand. –
Copy all of the following text. Then, in Max, select New From Clipboard.