Set focus on textbox in WPF

txtCompanyID.Focusable = true;
Keyboard.Focus(txtCompanyID);

msdn:

There can be only one element on the whole desktop that has keyboard focus. In WPF, the element that has keyboard focus will have IsKeyboardFocused set to true.

You could break after the setting line and check the value of IsKeyboardFocused property. Also check if you really reach that line or maybe you set some other element to get focus after that.

try FocusManager.SetFocusedElement

FocusManager.SetFocusedElement(parentElement, txtCompanyID)

In XAML:

<StackPanel FocusManager.FocusedElement="{Binding ElementName=Box}">
   <TextBox Name="Box" />
</StackPanel>

Nobody explained so far why the code in the question doesn't work. My guess is that the code was placed in the constructor of the Window. But at this time it's too early to set the focus. It has to be done once the Window is ready for interaction. The best place for the code is the Loaded event:

public KonsoleWindow() {
  public TestWindow() {
    InitializeComponent();
    Loaded += TestWindow_Loaded;
  }

  private void TestWindow_Loaded(object sender, RoutedEventArgs e) {
    txtCompanyID.Focus();
  }
}