These instructions are for a 'Windows' PC, using a standard keyboard and Mouse.
Open a fresh 'Tab' or new Browser window, and navigate to the page to which you want to paste a 'Link'.
'Left Click' in the URL / Search bar at the top of your browser, and the entire address line will be highlighted.
CTRL-C (that means hold down the Ctrl key - bottom left on your keyboard, and then press and release the 'letter C key', then let go of the Ctrl key). That will copy the URL (Uniform Resource Locator) or 'Address' to your clipboard.
Now go to the Tab or Window displaying the Forum, and do whatever you need to do to start making a post (either a reply, quote or new thread - whatever). When you reach the part of your message where you want to insert the link, click on the URL key, above where you are typing, and you will see the letters URL and /URL enclosed in square brackets, with the flashing text cursor between the two central square brackets (if it isn't between the brackets, then click between the brackets to make sure that it is!).
Then CTRL-V (same as CTRL-C, except that you press the letter V key on your keyboard) - this will then paste the contents of your clipboard (the Link address or URL) into the space between the central square brackets.
You can get a bit more fancy than that, for instance here is a standard 'Link': -
http://www.bbc.co.uk/news ...
... and here is an 'advanced' link: -
BBC News
The second type is achieved by putting a '=' sign after the first 'URL' but still inside of the first set of square brackets, and using 'Ctrl-V' after the '=' character, and then typing the description (in this case 'BBC News') in the space between the central pair of square brackets.
If you click on 'Quote' after reading all of this, you will see displayed in the text box, just what I mean...
You could even display a 'picture' or 'graphic' as your link if you wanted to, by inserting the 'img' tags from the bar above your text BETWEEN the central square brackets, and then putting a URL which points to the image between the 'Img' tags: -

...
Again, you will see how it looks in 'edit mode' if you click on 'Quote'
It is as well to use the Preview function before clicking 'Submit', because it's very easy to make a mistake, and computers are notoriously 'finicky' if you get a single character out of place!
Good luck.