In a similar way you'd do with a regular link.

A regular BYOND link goes in the form of byond://?src=[reference];var1=blah;var2=blah;... where [reference] is the reference to the object you want the link to use.

When you click the link, it will find the object [reference] (by using locate() in the background) and it will execute it's Topic() proc. You can then use that procedure to perform actions.

For HTML forms, it's only slightly differently. The code is like this:

<form action="byond://" method="get"> <input type="hidden" name="src" value="\ref[src]">

This will prepare your HTML form. Once a player submits the form it will execute the Topic() proc of src. The \ref text macro outputs a unique identifier for that object.

Keep in mind that you cannot change the method; you are bound to using the GET protocol, which has a limited ability to submit characters. Eventually the URL is going to become too large and it will trunicate the rest.
You can bypass this limit by submitting the form in chunks using some JavaScript.

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