You can check what resource names are available:

string[] names = this.GetType().Assembly.GetManifestResourceNames();
And then if Resource exists write correct name of Resource
                                            
Load the assembly where the resource is embedded, iterate through the GetManifestResourceNames to find the Fully Qualified Resource name which is then used to GetManifestResourceStream.
You can try 
var assembly = Assembly.LoadFrom(Assembly.GetAssembly(typeof(MainWindow)).Location);
string[] names = assembly.GetManifestResourceNames();
var stream = assembly.GetManifestResourceStream(names[0]);
                                            
Just an update to .NET 5. What helped me to find out the name of the resources to help fix my issue in .NET 5 was the following:
string[] names = Assembly.GetCallingAssembly().GetManifestResourceNames();
                foreach (var name in names)
                    Console.WriteLine(name);
                                
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