To link a discussion as a claim in another discussion, you use much the same technique as in Moving/Copying a Claim, but apply the steps to the thesis. Add the thesis to your clipboard, go to the discussion you would like to copy it to, and drag it to drop it into place.
Unlike a claim copied from another discussion, when you link an entire discussion, only the thesis appears (and not the claims underneath it). The thesis will appear as a hyperlinked claim, and the link will take you to the full discussion that has been linked.
For information on linking only part of a discussion, see Copying Claims Between Discussions.