The easiest way to add a quiz to your Shopify store is the Fyrebox block, which you place directly in your theme. No code and no HTML editing.
To have your quiz open in a pop-up rather than sit on the page, go to Theme settings > App embeds in the theme editor and turn on Fyrebox popup quiz. Paste your quiz link there and choose how and when it opens.
Older Shopify themes cannot use app blocks. If Fyrebox does not appear under Apps when you click Add block, paste the embed code from your Publish page into a page instead, as shown below.