When quickly creating a bot channels registration in Azure (I usually do this for testing something) You can select to create your own Microsoft App ID and Secret. If you do this then you can copy/paste them because you need them later on. But when you select “Auto create App ID and Secret” they don’t show you the secret in your bot settings.
If you want to find your App secret just look inside you notifications. Click on the name of the item you just created and you will get all the information used for creating it (also very handy if you ever want to create an ARM template or use the CLI). There you can find your secret under the “Inputs” section.