Hello Shena,
thank you for the kind words!
You are right: in the PDF24 Creator tool Create fillable PDF form, list boxes currently have no option for multiple selection, so they only allow one entry, like a dropdown.
Our online tool already has this option: https://tools.pdf24.org/en/make-pdf-fillable
Select the list box there and tick Multiple selection under its properties. You can also open the form you have already created there; existing fields can be edited, so you do not have to start again.
We will add the option to the PDF24 Creator as well.