What URL should be used for the App and for redirect for the Smartsheet new app registration form? I'm attempting to build a small app so I can learn about the smartsheet API, but I'm stuck at step 1. Should I just use the smartsheet developer portal? I was thinking about using Postman for this purpose, but I don't know how to use postman. I will be working with Python and just want to code a small tool to GET smartsheet data. So which rabbithole do I go down?
If you're just wanting to explore the Smartsheet API and test out some API operations by sending requests and viewing the responses, then using Postman would be the easiest route to do that. Using Postman only requires that you assemble the API requests properly (i.e., correct headers, URIs, parameters, request body) -- it doesn't require you to do any actual coding (e.g., Python). Looks like the Postman Learning Center contains some info that'll help you get started with Postman.
Regarding app registration within Smartsheet -- if you're just creating a sample app that you're going to use to explore simple API requests (and not a real app that you're ever going to actually deploy to production), I think you can use any URL for App URL and App Redirect URL (e.g., you could get by with just using https://www.google.com). However, keep in mind that building an app is a lot more work than just using Postman to explore the API -- if you're only wanting to explore the API for now, I'd recommend starting with Postman. Then once you're familiar with the API, you can use that knowledge to build an app that submits API requests and processes the responses.