I need a plugin like icon on my mac mail toolbar so that I can develop my custom plugin features.Is anybody knows how we can add a custom icon?

Question

I already created the .mailbundle file which is required for the mail plugin and it's also showing in my Mail preferences -> Manage Plugins. I just needed code that can add a custom icon on the mac mail toolbar in Swift, not in objective C. I had created a bundle named MyNewBundle, below screenshot showing the appearance of that .mailbundle in manage plugins.