Select cell B3 or any other cell. Click on Insert and select Drawing..., as shown in the following screenshot:

A drawing editor window will open. Click on the Text box icon and then click anywhere within the canvas area. Type Click Me. Resize the object to enclose the text only, as shown in the following screenshot:

Click on Save & Close to exit the drawing editor. Now, the Click Me image will be inserted into the active cell (B3), as shown in the following screenshot:

You can drag this image anywhere around the spreadsheet except in the menu bar.

If you click on the image, a drop-down arrow on the top-right corner will be visible:

Click on the Assign script… menu item. A script assignment window will open. Type greeting or any other name you like, but remember the name as the same name will be used to create a function in the next steps. Click on OK:

Now open the script editor in the same spreadsheet. When you open the script editor, a project selector dialog will open. You can close it or select Blank Project. A default function, myFunction, will be there in the editor. Delete everything in the editor and insert the following code:

function greeting() {
  Browser
}

When you type . next to Browser, the code completion hint will open as shown:

If you click on msgBox(String title, String prompt, ButtonSet buttons):String, then msgBox(title, prompt, buttons) will be inserted automatically.

In this code, Browser denotes that you are calling the Browser class from the Base (or basic) script services. msgBox is the Browser class's method with three parameters. The names of the parameters are self-explanatory. The title parameter denotes the title of the message box, prompt denotes your message to the user, and buttons denotes what category or sets of buttons you would like to include in your message box.

Now edit or replace the code with the following:

function greeting() {
  Browser.msgBox("Greeting", "Hello World!", Browser.Buttons.OK);
}

Click on the Save icon and enter a project name if asked. You have completed the coding of your greeting function.

Now, activate the spreadsheet tab/window and click on your Click Me button. An authorization window will open and you need to click Continue. In the successive Request for Permission window, click on Allow, as shown in the following screenshot:

You only need to do this once for this particular scope. The scopes will be shown in the concerned permission dialog/window. In this script, the scope is View and manage your spreadsheets in Google Drive. Above the scope, you can see the title Chapter 2 would like to:, which means your script project (project name Chapter 2) or application would like to get your permission for that particular scope.

As soon as you click Allow, the permission dialog will close, and your actual greeting message box will open as shown here:

Click Ok to close the message box. Whenever you click on your button, this message box will open.