There is a flag/banner (ribbon?) icon below posts (and comments), if you click the icon, the post (or comment) will be visible in your in-app saved items (bottom toolbar, looks similar to the save icon). I'll post screenshots.
Here's the icon to save a post/comment:
Here's the bottom ribbon icon to view saved items:
The screenshots make this post confusing for me, lol! Every time I edit I get confused about where to click! Sorry for the poor cropping of the images.
I couldn't remember the word for bookmark! Thank you! I tried to compensate by over describing, lol