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.
Go to Guest experience
Click on Custom codes
On the top right, click on Add new
You can then:
Name the custom code freely (for example "ask_for_flight", which means you will be able to use it in your templates under
%%ask_for_flight%%).Set the condition to "the guest is likely to fly".
Include the text asking your guests their flight number.
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.
Don't forget to include your custom code in your templates, where you want them.