programmatic textblock entry with linebreaks

Use Enviroment.NewLine

testText.Text = "Testing 123" + Environment.NewLine + "Testing ABC";

StringBuilder builder = new StringBuilder();
builder.Append(Environment.NewLine);
builder.Append("Test Text");
builder.Append(Environment.NewLine);
builder.Append("Test 2 Text");
testText.Text += builder.ToString();

You could just pass in newline \n instead of <LineBreak/>

helpBlock.Text = "Here is some text. \n Here is \n some \n more.";

Or in Xaml you would use the Hex value of newline

 <TextBlock Text="Here is some text. &#x0a; Here is &#x0a; some &#x0a; more."/>

Both results: