Thanks for asking.
The example website URL you have shared is using Pro Theme header builder navigation inline element. I also see that you are using Pro Theme. In that regards I suggest you to use header builder and you will be able to easily mimic the look and feel of submenu. Here is a screencast that you can take a look. https://cl.ly/1B0F3z363844
To get started with header builder, please take a look at following tutorial.