Permissions on application startup

Just ask for permission through scripting.

 void Start ()
     {
         #if PLATFORM_ANDROID
         if (!Permission.HasUserAuthorizedPermission(Permission.Microphone))
         {
             Permission.RequestUserPermission(Permission.Microphone);
             dialog = new GameObject();
             }
         #endif
     }

More documentation on this can be found in Unitys manuals. https://docs.unity3d.com/Manual/android-RequestingPermissions.html