There are a couple of ways to link to another site. You can insert a hyperlink using the chain link icon in the reply box, 4th from the left in the header or you can copy and paste a URL into the reply post.
To post a link you highlight the address bar at the top of the page you are on and then copy it, then if you are on your phone or tablet you do a long press in the reply box here and paste it into the box. That’s how I add links when I am replying to someone on here.
The patterns that I have don’t call for a marker. I was just wanting to make the toe’s rounded on my toe up socks. Which call for kfb increases. I was just confused this the term MRK. It’s not from a pattern but an article for a guide to use when making socks.
Reading the line below the photo in your original post, I’m going with the marker is keeping your spot for increasing. I did round toes once, didn’t like them much so haven’t done them again. YMMV of course since your feet aren’t my feet.