Executing viewmodels command on enter in TextBox

I know I am late to the party, but I got this to work for me. Try using Key="Return" instead of Key="Enter"

Here is the full example

<TextBox Text="{Binding FieldThatIAmBindingToo, UpdateSourceTrigger=PropertyChanged}">
    <TextBox.InputBindings>
        <KeyBinding Command="{Binding AddCommand}" Key="Return" />
    </TextBox.InputBindings>
</TextBox>

Make sure to use UpdateSourceTrigger=PropertyChanged in your binding, otherwise the property will not be updated until focus is lost, and pressing enter will not lose focus…

Hope this was helpful!