@PDHide

Can I ask for a 2024/2025 update to this answer?

I can't thank you enough for this helpful response. It was invaluable to me when upgrading to EtsyV3 in 2023. But since then my credentials were invalidated (2X) by Etsy and when you reset your password it invalidates your tokens, so you must do this process to reauthorize again and it is no longer working. There is now a warning message that says "Callback URL does not match Application Domain" underneath the "https://oauth.pstmn.io/v1/callback" entry in Etsy and when you try to "solve the captcha puzzle" it tells you "You are blocked" unless you enter a matching domain name in the callback URL.