wp popup pro css file in html

Hello,

i want to link to a css file in my html code which i put in my popup. Where do i need to put the css file on my server? i tried to put it in the wp-popup folder as well as in the css folder within the wp-popup folder – both didnt work.

thats how i linked it in the popup mask:
<link href=”popup-styles.css” rel=”stylesheet” type=”text/css” />

Thank you !

https://www.ads-software.com/plugins/m-wp-popup/