Depending on what you want to do precisely, there are several options:
(1) Display the <video> via the <surveypage>'s /stimulusframes.
(2) Display the <video> via a separate <trial> element and e.g. /branch to the <surveypage> containing your sliders afterwards.
<surveypage>s are special <trial> elements, i.e. you can use them in a <block> just like any regular <trial> element. However, you *cannot* run <trial> elements in a <survey>.