When you create a custom control within the same project and/or reference a dll containing any custom control, you will be able to utilize the Visual Studio Toolbox to drag and drop the control directly to the XAML/designer view.
After creating the control or adding the control library in a project, you need to build it first. Now, open any XAML page and navigate to the Visual Studio Toolbox. You will be able to find the control, as demonstrated here:
Now you can drag it to the place where you want to add the said control.
