Write the message as normal and put the words 'Click Here' where you want the link to appear.
Then highlight the words 'Click Here' and you will see the icon above your new message that looks like a chain and also one that looks like a broken chain will become active (bold). Click on the link one and then enter the address of the link.
i.e. if you go to another website and copy the address - especially e-bay - you can then paste it into the pop up window for the link. Then click 'Insert'. Job Done!

I've put the link below this sentance but it can be put anywhere in the text above on any words.

Click Here