You should only ask for your guest's flight number when it is relevant. If you decide to automate that part, you can create a custom code.
Name it the way you like (such as
Include the text asking your guests their flight number.
Set the condition to "The guest is likely to take a plane.".
Don't forget to include your custom code in your templates, where you want them.
If you have a multilingual setup, you should create one custom code for each language you support.
When your guest is likely to take a flight (because driving would be very long, or there is no other way to travel), we will include that block of text. If not, your guest will never see your message or your custom code.