Unfortunately this isn’t available in actual Muse.
For the time being, you have to use a service like Jotform.
I've already looked at those posts and others but they are not helpful as the links are broken
Oh looks like widgets.mu have shut their site.
We need to find some out of the box solution.
Being a creative person the ball is in your court Uwe.
As I already said: As far as I know, there actually is no widget out there, which provides form dropdowns. Jotform works fine, and in many cases, dropdown can be easily replaced by check boxes.
Widget.mu was the company of the former Muse engineer Andrew Hushbeck, but the company, like the homepage, doesn’t exist any more. The reason: https://widgets.mu