How to create custom keyboard shortcuts for google app script functions?

Apps Script only exposes server side events. Unfortunately, you cannot register client side events like keyboard strokes today. Please log an issue in our issue tracker

Very recently (April 2018) Google launched a macro recorder that includes a way to assign a keyboard shortcut to fire a macro and a way to import existing scripts as macros. See Google Sheets Macros

NOTE: This feature is currently being rolled out so it could take few weeks to be available for all. In my case it was available first on my personal Google account and since yesterday it is available on one of my G Suite accounts.