Since you use a dataset, which manages your dropdown (right?), then i would say to take a look onto the property-panel of your dataset which is connected to your dropdown.
There you should find SORT-OPTIONS.
But if you want to go a more dynamic way and generate it by code, maybe this post can give you some ideas…
Surely there are more examples out there to be found in the SEARCH of this forum.
I would have to take a look directly onto your setup to give you more advice, or even solve your problem.
CODE is directly visible, but your setup is only partially visible for me.