Rather easy, once you know how.  These instructions have been tested on Lotus Notes v6.5 & v8.5.

Step 1 – Select the Link Hotspot.
Highlight the text you want to create a link of.  In this case, it’s “Windows Mobility Center”.
Click on the Create / Hotspot Link Hotspot menu option.
Creating a web link in Lotus Notes - Step 1

Step 2 – Type your web address in.
In the Type field, ensure URL is selected.
In the Value field, type or paster your web page link.
Then click the “X” in the top right corner of HotSpot Resource List box.
Creating a web link in Lotus Notes - Step 2

Step 3 – All done!
Your highlighted text is now a weblink.
Creating a web link in Lotus Notes - Step 3