Could you just include that on the verso page, rather than on the cover itself?
If you really want the text on the image, then you could do something like this in the Front Matter section that defines the front cover matter:
\periph front|id="coverfront"
\zgap|-1.2cm\*
\mt1 \zvar|maintitle\*
\zgap|8cm\*
\mt2 \zvar|subtitle\*
\zgap|4.4cm\*
\mt3 \zvar|author\*
\qr Designed by Freepik
\vfill
\endgraf
\periph spine|id="coverspine"
... etc.
Make sure you set the style for \qr to be the right color (white?) and adjust the \zgap values (creating vertical space) to make it sit at just the right place.