If you go to your articles in your profile your coop page is there. I'm on iPad right now so I just hold down on the title of the coop page and I get this drop down box to copy the URL. It's the same if you are on iPhone.
Click copy then go to your personal page and hold down in the add coop box. You should get the option to paste like this.
Your URL should then be in your box. Hope this helps
I found this from (Yorkshire Coop moderator).