Field Customizers are the latest and greatest feature to be provide within the new SharePoint Framework. If you are like me, you immediately opened up command prompt and VS Code, grabbed the new Yeoman template and started testing them out!
The logical starting place would be going to the MS Docs for SPFx Field Customizers. If you go through the tutorial, it's pretty straight forward. However, unlike SPFx webparts, you cannot debug these field customizers locally. You actually have to create a site and pass a url similar to this.
You'll notice a couple parameters. The first being loadSPFX=true, this is because the SharePoint framework isn't loaded by default. The second is a location to your localhost manifest.js file which is running on your local machine when you use gulp serve --nobrowser.
Now, you are so excited you breeze through the Microsoft Docs but there is one thing you notice... it says
"Append the following query string parameters to the URL. Notice that you will need to update the id to match your own extension identifier available from the HelloWorldFieldCustomizer.manifest.json file:
Navigate to your manifest.json file and grab the ID and pass it in your query string parameters where it says fieldCustomizers=...., and replace the ID with the ID from your manifest file.