This post shows you where to get Client Secret from Microsoft Graph if you want to use Postman, https://github.com/microsoftgraph/microsoftgraph-postman-collections. The link never showed you how to get that. If you follow the link you will arrive on the following screen and get stuck.
You must already have Application / Client Id from at app.dev.microsoft.com.
Click on the App then the follow screen will pop up. Choose View this app in the Azure Portal.
Your browser will bring you to here.
Click on Certificate $ secrets. This is where you can get the Client Secret.