Here you go:
<script>
var Webflow = Webflow || [];
Webflow.push(function () {
document.getElementById('w-dropdown-list-1').classList.add('w--open');
});
</script>
Caveat: Since the dropdown is open by default then closing it for the first time takes two clicks. Because I haven’t overwritten the default Webflow’s interaction to open it on the first click and then close it on the second click.
How to use:
-
Paste this code in the body tag.
-
You need to replace w-dropdown-list-1
with an ID of the dropdown list you want to show by default. Do you know how to use Dev Tools to locate the ID? If not. Then add a custom ID manually within Webflow to the Dropdown List (the one nested within the parent Dropdown) element:

and use that ID in the code instead of w-dropdown-list-1
.
I’ve tried this on the default dropdown and it works. But your specific case might have some other requirements that might not work with this. Let me know if that’s the case.
Note, test the code on the published version. It won’t run in preview mode or the designer.