For outbound problem notifications you'll primarily see stuff assuming the endpoint you're sending too consumes json and you'll notice also that the UI complains if your entry doesn't have valid json.
So first thing you may need to do is add a Content-Type header to the notification (can add application/xml etc...) and then Dynatrace understands you're not trying to send json. After that though you just need to make sure it's valid xml so that your endpoint can process it (you can use some tools or editors to validate it) and you can use the curly braces to include placeholders just like for json, but apart from that more depends on what you're integrating with than any examples that would be specific to Dynatrace.
Yep! It'll show you which ones are available, just know that the placeholders will be filled with actual problem data when they are detected. Apart from that just make sure the xml is valid and is in the format that your endpoint expects: