How do I set a keybinding for an extension in VSCode?

If you open your extension's information window, you might see a Contributions tab, and in there you might see a Commands list.

From there you can find the command that you want and bind to it in your keybindings.json file or File -> Preferences -> Keyboard Shortcuts

[
    {
        "key": "ctrl+enter",
        "command": "command.execute",
        "when": "editorTextFocus"
    }
]

In case somebody is writing their own extension for VSCode, you can set up a default key binding for your commands using the keybindings prop alongside with commands inside contributes prop. Example setup in package.json of a sample project inited by Yeoman yo code command:

{
    "name": "static-site-hero",
    "displayName": "Static site hero",
    "description": "Helps with writing posts for static site generator",
    "version": "0.0.1",
    "engines": {
        "vscode": "^1.30.0"
    },
    "categories": [
        "Other"
    ],
    "activationEvents": [
        "onCommand:extension.helloWorld",
        "onCommand:extension.insertLink",
        "onCommand:extension.insertFigure"
    ],
    "main": "./extension.js",
    "contributes": {
        "commands": [
            {
                "command": "extension.helloWorld",
                "title": "Hello World"
            },
            {
                "command": "extension.insertLink",
                "title": "Insert Markdown Link to File or Image"
            },
            {
                "command": "extension.insertFigure",
                "title": "Insert HTML figure"
            }
        ],
        "keybindings": [
            {
                "command": "extension.insertLink",
                "key": "ctrl+alt+l",
                "mac": "shift+cmd+f"
            },
            {
                "command": "extension.insertFigure",
                "key": "ctrl+alt+F",
                "mac": "shift+cmd+l"
            }
        ]
    },
    "scripts": {
        "postinstall": "node ./node_modules/vscode/bin/install",
        "test": "node ./node_modules/vscode/bin/test"
    },
    "devDependencies": {
        "typescript": "^3.1.4",
        "vscode": "^1.1.25",
        "eslint": "^4.11.0",
        "@types/node": "^8.10.25",
        "@types/mocha": "^2.2.42"
    }
}