Coming across this a fair bit, and as we're getting busier it's getting more difficult to correct people's posts.

1. Copy and paste the link into your post then highlight it all.

2. Look for the little chain link icon above the text box.

3. When you press it, in the little box that appears paste your link into the top and bottom boxes

4. Change the middle box to 'open link in a new window' and press Insert

5. When successful it will underline your link

Alternatively

If you post in the quick reply, when you paste in a link it should do this automatically for you, but this might be device dependant.

Any questions, reply below