Go to notification list >> Edit Notification >> Click tab

The first field that you come across in the click tab is the CTA label and link. Toggle it to on to display your call to action in the notification.  

You can get to further customize the CTA label. You can change the font and the style of the font. Choose your own color for the text field. Also, you can change the background color of it through the CTA background color option.

Set CTA label action once it is clicked. You can either add a custom URL and it will be redirected to the URL once the CTA label is clicked on. Or else you can choose the open collector form to capture the data.  Further, You can customize the collector where you can write the collector text & set the capture field as well.

Or else you can simply choose the None of the above options if you don’t want any action to take place once the CTA label is clicked upon.