Cannot use the kill_background function

Hello,
I try to embed my poll on my website. It works fine except for the background. I would like to make it transparent and I tested the following (from your tutorial):

<iframe frameborder="0" allowtransparency="0" src="https://pollunit.com/polls/MEMBERLINK?embed=1&embed_options[kill_background]=1"> </iframe>

However, I cannot save the change as it consider I got an error in my piece of code.

Thanks for your help
have a nice day

Can you send the link to your website where you want to integrate PollUnit to support@pollunit.com. Then we take a look at it.