Well, I’m assuming you’ve already defined $comboBox2 in your GUI. If not, I’ll be honest - this is a Q&A forum, and it’s not really a place for a tutorial on something as complex as GUI programming. You might consider using a tool like SAPIEN PowerShell Studio, which simplifies creating the GUI and defining the controls. They also have a very robust support forum to answer questions.
Ditto on Sapien’s PSS, I been using that for years now. Yet, it’s a annual sub cost. well worth it though.
What you are asking has been a thing in GUI (PS notwithstanding) for a long time. SO, not a PS specifc thing but how you handlet the call to your data cource and the form. E,g.:
You don’t say how you are doing this GUI, WPF / XAML, but all-in-all, the approach is the same.
Here is a sample of this in action with PS and XAML. It’s not pretty, but it works, thus gives you the idea of what you’d want to do. Remember there are always more elegant / efficient ways to do X or Y. You just need to take your CSV and split the data on load / read into the comboboxes.