Differences across 2013-2016 versions, but articles written as if they're the same
Please verify options or identified items are actually exactly the same before posting documentation, tech, support articles showing that something is exactly the same across versions.
I and users have run into this issue on a number of articles. This causes confusion and frustration, as it makes you expect options or to see configuration settings/have them available, but they're not.
The top of the document shows "Applies To: SharePoint Online, SharePoint Server 2016, SharePoint Foundation 2013, SharePoint Server 2013 Enterprise, SharePoint Server 2013, SharePoint Online admin center"
But the custom message option identified for "Access Requests Settings" is nothing like the one shown, using SharePoint Server 2013. There is no ability to include a custom message. All that is actually there is to have it on/off and add the email.