Unfortunately, the surrounding program code is missing here to give an exact answer. However, there are a whole range of questions and answers on the problem "dropdown".

https://stackoverflow.com/a/79037245/22768315

GTK 4 drop down signal "activate" not working

https://stackoverflow.com/a/79225876/22768315

In addition, there are also examples of how to create a list store:

https://stackoverflow.com/a/78291720/22768315

https://stackoverflow.com/a/77619798/22768315

Maybe that can help. Otherwise, please provide a meaningful script.

Have fun programming.