How to insert newline into MS Teams markdown?

You didn't provide any code, so it's hard to say exactly what you're trying, but if I set the text value in the card to this:

"text": "text": "There is a problem with Push notifications.<br><br>They don't seem to be picked up by the connector."

This how it's rendered:

If you're trying to just send a connector card w/ text in markdown, there's no need to enclose the text in triple backticks (unless you specifically want preformatted text). We automatically treat the text in a connector card as markdown, unless you set the markdown property to false.

To add line break in markdown, end the line with 3 spaces + linebreak. For example:

msg.text("Line 1   \nLine 2")

Connector cards sent by a bot:

If you added those triple backticks to get preformatted text, then you need to place the backticks on their own lines to allow multiline text. For example:

msg.text("```\nLine 1\nLine 2\n```")

Connector cards sent by a Connector:

If you're developing a connector or using the incoming webhook connector, that uses a different parser that doesn't convert "```" markdown to a <pre> tag. It's best to just fall back to HTML, as Bill suggested below. For example:

msg.text("<pre>Line 1\nLine 2</pre>")

HTML tags work inside a <pre> element, so as you found out, this is equivalent:

msg.text("<pre>Line 1<br>Line 2</pre>")