I guess you would have to define it the same way you defined it in AC15. Can you export your Keyboard Shortcuts from AC15's Work Environment Dialog and import it in A19?
Alternately, if you go to the Work Environment Dialog, select Keyboard Shortcuts item on the left, the right will show all the shortcuts. Select "All commands in alphabetical order" in the "List" field. Type "Zoo" and it will take you to the "Zoom In " command. Assign any keyboard shortcut to it, then OK the Dialog.
Then maybe you can use a mouse macro software to map your mouse button to that key you assigned to the Zoom In command.
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