This issue can be approached in two ways that I can think of.
- In case you are using the Traffic Manager to route the traffic to different regions, in that case only if you publish you code for API on a WebApp hosted in a separate region for eg: SEA with the same reply URLs, I believe would work. When the user sends a request from SEA region, the traffic manager would itself route the traffic to the WebApp hosted in the SEA region and if the request is made from US region then the Traffic Manager would route the traffic to the WebApp hosted in US region.
- Secondly since you have two WebApps (one in US region and other in SEA region), you can specify two different Reply URLs for each of the WebApps. Then under the single App Registration in AAD, that was done earlier for the US hosted WebAPP, you can add the new reply URL for the SEA hosted WebApp. So, the app registration in AAD for the WebApp would have two reply URLs. Now when the user tries to login from US region, in the request to AAD, it would add the reply URL for the WebApp hosted in US region and based on that Reply URL, AAD would post the reply on that same reply URL, provided that URL is registered in the App Object present in AAD.
Hope this helps.
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