Well, you can roll your own dialog (Template or Prompt dialog) and display, for example, a button with underlined text or just regular text. When you get the callback, you can invoke a browser.

The complications are a few
1) invoking a browser is really more complicated than simply calling system(). Netscape Navigator has an entire properties-based protocol for invoking the browser with a particular URL. Having implemented the application half of the conversation, I can tell you that the protocol was designed for the convenience of the browser folks. Doing an exec() or a system() call is lots easier but doesn`t let you know when, for example, the browser has failed to come up or isn`t installed properly or is already present.

2) I bet that what you really want to do is to display a bunch of text with a particular string underlined, and then display a browser with a URL associated with that string. This is really a more general problem of displaying HTML or text. One solution may be to wrap the XmHTML widget into a dialog. You can also use the simple browser that XmHTML comes with.