By default, EPUB opens with FBReader in Tirwal. If you are happy with that engine, just press '6' to create a bookmark.
IMHO, the Coolreader engine is much more flexible. To get EPUBs to open in Coolreader, I just rename the file by adding an ".fb2" file extension which forces the firmware to treat it as a FictionBook 2
file (my favourite file format). With the book open in Coolreader, just follow my previous post re bookmarking. You will also have greater choice in adjusting font type & size, displaying time, accurate page numbering etc.
As for MOBI files, according to Tirwal's posts here
(you need Google Translate if, like me, you don't speak Spanish) the firmware uses the original Jinke viewer for DRM MOBI. I always convert the few MOBI files I get to .fb2 format, using Calibre &/or FictionBook Designer software, so I'm sorry but I have no knowledge re bookmarking MOBI files.
Hope there's something useful for you here.
BTW Tirwal's firmware is up to version 3.1, available from the link above in paragraph 3.