The latest version of Xamarin seems to have it build in.
Here are the steps that I followed
1.) In the Xamarin Studio , go to the main app, and add the Crashlytics API key in the Build -> Crash reporting section. Build and run that app. The evidence suggests that is enough to get data sent to Crashlytics. I saw that it knew about older versions of the app I had built and ran weeks ago with the API key already in.
2.) Go to Xcode and create a new project with the same name as the Xamarin project, com.{company}.{appname}. (You probably need to have that profile downloaded.) I did it in Swift by the way with no issues.
3.) Go to crashlytics.com com and go to settings -> Apps (or https://www.crashlytics.com/onboard)
4.) You will get an app to download and run. The app walks through the steps of adding
A.) A build phase run script
B.) Code to the application main Crashlytics.startWithAPIKey(“nnnn”)
5.) Run your Xcode version of the app
The Crashlytics web page popped up immediately.