Example using Hyperlink in WPF

If you want your application to open the link in a web browser you need to add a HyperLink with the RequestNavigate event set to a function that programmatically opens a web-browser with the address as a parameter.

<TextBlock>           
    <Hyperlink NavigateUri="http://www.google.com" RequestNavigate="Hyperlink_RequestNavigate">
        Click here
    </Hyperlink>
</TextBlock>

In the code-behind you would need to add something similar to this to handle the RequestNavigate event:

private void Hyperlink_RequestNavigate(object sender, RequestNavigateEventArgs e)
{
    // for .NET Core you need to add UseShellExecute = true
    // see https://docs.microsoft.com/dotnet/api/system.diagnostics.processstartinfo.useshellexecute#property-value
    Process.Start(new ProcessStartInfo(e.Uri.AbsoluteUri));
    e.Handled = true;
}

In addition you will also need the following imports:

using System.Diagnostics;
using System.Windows.Navigation;

It will look like this in your application:

If you want to localize string later, then those answers aren't enough, I would suggest something like:

<TextBlock>
    <Hyperlink NavigateUri="http://labsii.com/">
       <Hyperlink.Inlines>
            <Run Text="Click here"/>
       </Hyperlink.Inlines>
   </Hyperlink>
</TextBlock>

In addition to Fuji's response, we can make the handler reusable turning it into an attached property:

public static class HyperlinkExtensions
{
    public static bool GetIsExternal(DependencyObject obj)
    {
        return (bool)obj.GetValue(IsExternalProperty);
    }

    public static void SetIsExternal(DependencyObject obj, bool value)
    {
        obj.SetValue(IsExternalProperty, value);
    }
    public static readonly DependencyProperty IsExternalProperty =
        DependencyProperty.RegisterAttached("IsExternal", typeof(bool), typeof(HyperlinkExtensions), new UIPropertyMetadata(false, OnIsExternalChanged));

    private static void OnIsExternalChanged(object sender, DependencyPropertyChangedEventArgs args)
    {
        var hyperlink = sender as Hyperlink;

        if ((bool)args.NewValue)
            hyperlink.RequestNavigate += Hyperlink_RequestNavigate;
        else
            hyperlink.RequestNavigate -= Hyperlink_RequestNavigate;
    }

    private static void Hyperlink_RequestNavigate(object sender, System.Windows.Navigation.RequestNavigateEventArgs e)
    {
        Process.Start(new ProcessStartInfo(e.Uri.AbsoluteUri));
        e.Handled = true;
    }
}

And use it like this:

<TextBlock>
    <Hyperlink NavigateUri="https://stackoverflow.com"
               custom:HyperlinkExtensions.IsExternal="true">
        Click here
    </Hyperlink>
</TextBlock>