Go to the page you want to link to and copy the address line.
Then paste that into, for instance, the message box, like this one.
That line will then start with
http://etc,etc.
Before the http type < a href=
Don't use any spaces as I have in the line above, except between the 'a' and 'h'. I've had to use one between the symbol and the 'a' to prevent it appearing as a link.
After the line you've pasted type >and then whatever words you want to appear in blue underline.
And end with < /A> But with no spaces.
For example a href=http://www.gobbledegook.com>gobbledegook< /a>
I have deliberately missed off the < before the 'a' and put a space in the ending code so that it doesn't appear as a link.