We want our participants to download our app in order to goto the contest. Can this happen?
I am not a native Android or iOS developer, so I can only guess here. As far as I know it is possible to include HTML code in apps. Since you can embed PollUnit with an iFrame on your website, this should theoretically also work with an app.
To allow PollUnit to connect to the website, the host of the website has to be allowed in the PollUnit settings (Content Security Policy). If this is a problem in app development, you should clarify with your developers. If the developers need any information, please write here again.
I think most mobile devices provide a "web view" component. This could point to the URL of the contest and then people can vote there.