I am using ArcGIS Enterprise 10.6.1. I was trying to add custom widgets, and in order to do so, I have to click on "My Content - Add Item - An Application". Once I click into an Application, I don't see an option as "Application Extension(App Builder)" in the "Types" option. I read that Custom widgets are available starting the version of ArcGIS Enterprise 10.5. Why do you think that type is not showing while creating an application. Because I want to add a customized widget into my portal?