The Blackboard's toolbar makes adding a link to the contents of any textbox -- be it a discussion board post, announcement, email, anything -- very easy. First you'll want to write your post or item which will include the link. Then, open a new browser tab (CTRL+T), and find and copy your URL. For this example, I've pulled the permalink to a journal article from the Library's catalog:

Next, highlight the text you want folks to click on to open the link (in the example below, "DiPaolo's leadership article" will be our link text), then click the link button in the toolbar, which looks like a couple links of chain.

If you don't see it immediately, click the + button, then choose "Insert / Edit Link":

Then, paste (CTRL+V) your link url into the "Link URL" field and click the INSERT button.