How to Line Break or new line in XAML

You can use preserve. It includes all whitespace, so inputting the exact string you want would involve messing up your indentation, but this will work:

        <HyperlinkButton xml:space="preserve">This is line one.
This is line two.</HyperlinkButton>

You've got options. For example;

<HyperlinkButton Content="Line One&#10;Line Two"/>

or

<HyperlinkButton>
  <HyperlinkButton.Content>
    <TextBlock>
      <Run Text="Line 1"/><LineBreak/><Run Text="Line 2"/>
    </TextBlock>
  </HyperlinkButton.Content>
</HyperlinkButton>

Hope this helps.

Addendum: You can do this stuff in basically anything. WPF, Silverlight, UWP, whatever. It's not WP specific.