If you want the user to be able to focus a specific field with an ALT+Key – eg. ALT+F, you can do it the following way:

<Label Target="{Binding ElementName=Username}">Enter _Username:</Label>
<TextBox
Name="UserName"
Text="{Binding Username, Mode=TwoWay, UpdateSourceTrigger=LostFocus}">
</TextBox>

• Target of Label associates it with the TextBox.
• Underscore _Before a letter makes it accessible via the ALT+Key

If you are from the Web World, you are used to associating labels with inputs via the ID-attribute. Target looks similar.