Can you elaborate more on your scenario? What do you mean with telerik Form?
This textbox that is in the UserControl - is this user control created before button is clicked or after?
What is the main goal here?
If you need just to set TextBox.Text property then you can instantiate the usercontrol set x:Name in the textbox inside the usercontrol and then expose the textbox as a property. Then on button click you could set the textbox text.
If you need to pass complex data if should probably use the MVVM pattern and bind the textbox text property to some property of the data context.
Let us know if you need more information.
All the best,
the Telerik team
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