Great to see it working
You can remove the name from the select 'cause you do not need to send it to SimBrief.
<select id="dropdown" onChange="ChangeFormValues()">
When you submit a form, everything you gave a name will be passed with its value (if it is enabled of course). We can not disable the dropdown here 'cause we need it, but we can avoid sending it to SimBrief by removing the name="" tag
Just a little trick for you.