Need help with a custom format script for a dropdown menu

Hi! I'm creating a purchase request form for my department. This form has a dropdown menu with different department accounts/numbers listed for them to choose from. On the dropdown, the text is formatted as-

[ACCOUNT #] - [ACCOUNT NAME] 

-and I have the account number as the export value. I currently have the below custom format script so that when the account number/name is selected, only the account number (listed as the export value) will populate:

event.value = event.target.getItemAt(event.target.currentValueIndices, true);

My problem is, that I also want to be able to insert custom text in addition to the dropdown options. When I currently try to enter custom text, it auto populates the closest account number listed in the dropdown, despite having the custom text option clicked. I understand this is likely due to the custom format script. Is there anything I can add to the script that will also allow custom text to be entered? 

Thanks!