For this you don't even need any special HTML input format. The default Filtered HTML already allows the HTML <a ...> tag, which is all we need.

The syntax for a link is as follows:

<a href="put the destination web address (URL) here">put the clickable text here</a>

Here is a simple example:

To get to the elephanttrust.org web site, please click here.

The code to achieve this is:

To get to the elephanttrust.org web site,
<a href="http://elephanttrust.org/">please click here</a>.

Hans