Secondary Title in Generatepress

Hi there, I have used a plugin called Secondary Title to add a secondary title field, however what I’m trying to do is place it inside an element which I am using as my header.

I tried using {{custom_field.secondary_title}} but that did not work. The PHP they tell me to use in the theme is <?php echo get_secondary_title(); ?> — Is there any way I could get the subtitle into an element (or another method that would allow me to do so)?

I don't mind switching plugins, or adding something to my child theme's functions if that's what is necessary.
(It would be a bonus if the field appeared under the actual title instead of in the sidebar, but that's not a big deal.)

Thank you!