How can i create a new combobox on runtime in javafx?

I´m new to Javafx and im trying to get into it. I want to creat an Application which is based on a users choose from a Combobox.
For example: The user has to choose a country from the combobox. After he selected a country, I want to show a new combobox with a selection of cities from the choosen country.
Now my Problem is: Based on the first choose there will be a sql select who gets me the data from a oracle database for the value of a new combobox. I´ve tried a lot but found no way how to add the new combobox after the select.
I hope one of you could show me how I can create a new combobox on runtime to the scene or an alternative way how to solve my problem...
The most important thing is the user should just be able to see one combobox at the start and with each choose he makes an additional combobox will appear.
Many thanks in advance for your help

Best regards Banki