How to a add a command to a WPF TextBlock?

You can use a InputBinding.

<TextBlock Text="Hello">
    <TextBlock.InputBindings>
        <MouseBinding Command="" MouseAction="LeftClick" />
    </TextBlock.InputBindings>
</TextBlock>

Edit: Hyperlink is probably worth a mention too.

<TextBlock><Hyperlink Command="" TextDecorations="None" Foreground="Black">Hello</Hyperlink></TextBlock>