The attached SpellCheck.IsEnabled property allows you to add spellcheck support to the TextBox control. Set it to True to enable it. Let's add a multiline textbox in the UI with this feature enabled:
<TextBox TextWrapping="Wrap" AcceptsReturn="True"
Height="60" VerticalScrollBarVisibility="Auto"
SpellCheck.IsEnabled="True"
Margin="10 5" />
Now run the application to have a window with a multiline text-input field in the UI. Enter some text with some spelling mistakes. You will see that the wrongly spelled words get highlighted with red underline. Right-click on it to see a context menu, which suggests words from the dictionary.
As shown in the following screenshot, select the one that is best suited in this context:
