In a custom module, how can I change its CSS class using on-click?

[ErisLoona]

Heyo! So basically I'm trying to make a start button that opens fuzzel. However, while fuzzel is running I want the button to look "pressed".
So I have the script:

#!/bin/bash

printf '{"class": "active"}'
echo -en "test\ntest1" | fuzzel --dmenu
printf '{"class": ""}'

set in the on-click option. The problem is that it looks like the custom module only takes output from the exec option, however I don't really see how I could make what I want while using exec.

Am I missing something? Or is there another way to just make a button?