Add subtitle to Portfolio Items?

Hello!

I wondered if I can add a subtitle to my Portfolio Item.
This is my Portfolio Item: https://www.mariahelenafineart.com/portfolio/red-stag/
And I would just like to add a short line of text immediately underneath, in the same way as you can do on Pages.

I have downloaded the Subtitles plugin by Professional Themes, but no option is appearing on my page, and I have checked the Screen Options at the top.

I have no coding experience so if this is a coding-related fix, please be gentle!
Thank you in advance for all and any help proffered!

Maria