opening a link in a frame

I’ve looked at the few posts I’ve found on this subject and nothing I’ve tried seems to work. I downloaded an iframe plugin and I currently have installed “page links to” plugin. I can’t get anything to work.

I have a site at https://www.walkingtrivia.com. I’m using the Adventure theme. I’m trying to get my teespring store to open up in the gray “post” that shows up in the brick wall area. I can’t seem to get it to work. I put the store url in the place where it says to put one for the plugin. Nothing.

Would be very grateful for any help. I even paid a guy on fiverr to help and he couldn’t figure out how to do it.