Set focus on a textbox control in UserControl in wpf

Another option that you have is to create a bool IsFocusedproperty in your view model. Then you can add a DataTrigger to set the focus when this property is true:

In a Resources section:

<Style x:Key="SelectedTextBoxStyle" TargetType="{x:Type TextBox}">
    <Style.Triggers>
        <DataTrigger Binding="{Binding IsFocused}" Value="True">
            <Setter Property="FocusManager.FocusedElement" 
                Value="{Binding RelativeSource={RelativeSource Self}}" />
        </DataTrigger>
    </Style.Triggers>
</Style>

...

<TextBox Style="{StaticResource SelectedTextBoxStyle}" ... />

Note that at times, you may need to set it to false first to get it to focus (only when it is already true):

IsFocused = false;
IsFocused = true;

This is similar to Sheridan's answer but does not require focus to be set to the control first. It fires as soon as the control is made visible and is based on the parent grid rather than the textbox itself.

In the 'Resources' section:

    <Style x:Key="FocusTextBox" TargetType="Grid">
        <Style.Triggers>
            <DataTrigger Binding="{Binding ElementName=textBoxName, Path=IsVisible}" Value="True">
                <Setter Property="FocusManager.FocusedElement" Value="{Binding ElementName=textBoxName}"/>
            </DataTrigger>
        </Style.Triggers>
    </Style>

In my grid definition:

<Grid Style="{StaticResource FocusTextBox}" />