Type the whole address, scheme included
Paste the link exactly as the browser shows it, https:// and all. If you leave the scheme off and what you typed looks like a host name, the generator adds https:// for you; if it does not look like a host (a phrase with spaces, say), it is encoded as plain text and the scan shows words instead of opening anything. Prefer https over http: some mobile browsers interrupt an http page with a security warning, which is a poor first impression for a code somebody just scanned off a poster.