for the purpose of ITA you only need to model a website, it does not need to be 100% functional. 'prototype'
Assuming this is the case, it should be satisfactory to merely have just have a webpage with a form that has the field 'username' and 'password'. Upon pressing submit, the user can then be redirected to the 'member-only, closed access pages'. If a user was navigating your website by clicking on links only, they should not be able to land on these 'member-only, closed access pages' without having gone through the login page which contains the form.
I don't remember what the automatic dialog thingy for dreamweaver is like, but the form would be something like
<form name="loginFORM" method="POST" action="/members/index.html">
<label for="username">Username</label>
<input type="text" name="username" id="username" value="" tabindex="1" />
<label for="password">Password</label>
<input type="password" name="password" id="password" value="" tabindex="2" />
<input type="submit" name="login" id="login" value="Login" />
<input type="checkbox" name="remember" id="remember" value="1" tabindex="3" /> Remember Me?</input>
</form>
You could alter the value for 'action' to be any webpage on your server. You probably wouldn't need to be using POST since there isn't any actual server-side code.
Some of the other attributes I included are a bit unnecessary for minimal function as well (i.e. all the name and id attributes, but they do come in quite handy once you start applying CSS and all that). The label stuff is somewhat unnecessary as well.
If you're bored and wanted to take the prototype website thing one step further, you could implement a fake database kind of functionality through javascript (have it check if the username and password entered into the form are equal to a predefined username and password in the code). Obviously that's not necessary for IT Apps and not something you'd ever use for a functioning website.
If you wanted this login form to actually be functioning, you'd most likely set action to land on a server-side scripting enabled page (php in combination with a database for example, like Observer7 mentioned above). The login form itself would probably be a php page as well, and you could set up get some fancy validation going on - also something that can be done solely through javascript in some cases. Depending on the extent of validation you're running - if you just wanted to check if the user hasn't entered funny symbols or gone over a predefined character limit, yeah that's do-able in javascript, but if you wanted to check if a username existed in the database. Kind of like those check availability things when you register on websites is what I'm thinking of, then that's something you'd need server-side scripting + a database for] (once again, you could fake that functionality in Javascript [and once again, that's not a necessary thing to do for a prototype website, just kind of saying that it could be done if you really wanted]).