It depends what you want to link to. I like this website: https://www.w3school.../html_links.asp
For modules I have made for e-Sword 12.x I have used two kinds of links:
The following appears as superscripted [n] and the message appears upon mouseover:
<a href="The information that shows in a tooltip"><not>[n]</not></a>
The advantage of this is that a separate Commentary module is not needed to display the notes.
The disadvantage of this is that the tooltip note cannot display formatted or utf-8 text (such as Greek or Hebrew).
In the module Details table under Information I place a link to my e-mail. What appears is 'agsper652' and what happens upon click is that your email client (Windows Mail or whatever) opens up and has put in my email address. It has the following format:
Link to outside website
One can also make links to outside websites such as the one I suggested above.
What shows up is 'See w3schools', upon mouseover is a message (on my computer anyway) that says 'CTRL+Click to follow link', and when I push down the CTRL key on my keyboard while I click on the 'See w3schools', then my web browser (Firefox) opens the website. It has this format:
<a href="https://www.w3school.../html_links.asp">See w3schools</a>
I haven't used this outside website link, but it can be useful. For example, in a Commentary module it might take one to a website with a reference book or a map or a photo of the location.
I hope this is helpful for what you wish to do.