Thanks @v-yamao-msft,
The other thread was very helpful.
I finally got this working and I want to share the solution and some things I have learned about MS Flow and testing the ArcGIS REST API using Postman. Maybe you seasoned programmers know this stuff, but I sure didn't. Ultimately, and most importantly it is possible to use the MS Flow HTTP trigger with the ArcGIS REST API to edit features, but it is a little tricky. The most annoying thing I found (and I'm not sure if MS Flow is at fault or the ArcGIS REST API) is almost always the response from the API will be 200 (Success), even though it is not successful. Below is a malformed request, but you can see we get a 200 back. It should respond back as a 400:
For awhile, I thought that my Flow was working properly, until I realized my features were not updating. Lesson learned, do not trust or rely upon feedback from MS flow. This little green box is deceptive.
I also learned some valuable info about Postman. I assumed that pasting in the raw sent request from the Postman Console into the body of the Flow HTTP trigger would work. (Because hey, the request works great in Postman - everything updated as it should). But even in "raw" mode, the console doesn't actually show the true request that was sent to the API. However, the "Generate Code Snippet" tool in HTTP mode does. This ultimately helped me figure out what to paste into the header and body portion of Flow.
Ultimately, pictured below is how you would format your header and body (configure your token for authentication under the "Show advanced options" at the bottom) to successfully POST to a feature using the ArcGIS REST API to update attribute features. The main trick is defining your boundary (unique string of characters which separates your different forms in the body) in the header (in my case I just used what Postman auto-generated, but it could be anything). I hope this saves you some time if you want to use MS Flow and the ArcGIS Rest API.
