Hi,
Welcome to Microsoft Q&A!
You could double-click the MainPage.xaml, you will see the XAML designer in window. As for Toolbox, you could select View option and open Toolbox, which displays many of the controls that you can use in your app, when you double-click the TextBox control, this XAML is added to the XAML view, you can also drag the control from the Toolbox to the XAML designer. Please refer to this official document to know more, such as creating an event handler and setting the control properties.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.