Appregnew

On the Site Collection were you would like to add the App, append the following URL: "/_layouts/15/appregnew.aspx"

E.g. Your Site collection URL is: https://sharepoint.local/sites/meetingmanager then the full URL would be:

Hint: While you register the App on this Site Collection, it can be added to any other Site Collection within this Web Application as well.

On following values are available:

App Id: Later on this will be referenced as Client Id as well. App Id and Client Id are the same. Click on "Generate" to create a new GUID. This GUID we'll use across your SharePoint (so it goes beyond this Site Collection)
App Secret: Click on "Generate", but there's no need to remember it, as it isn't needed in on-premise installations
Title: Solutions2Share Meeting Manager
App Domain: This is the URL of your "Web Meeting Manager" without protocol, e.g.: meetingmanger.contoso.com
Redirect URL: This it the same URL as App Domain + protocol and port, e.g. https://meetingmanger.contoso.com

Confirm using "Create" - remember all values!