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All you have to do is give the individual fields an ID (the form doesn't need one) .. and refer to those ID's.
For example, consider the following code:
<input type="text" id="mytextfield" \>
<input type="password" id="blah" \>
To refer to the first field you use:
Likewise, to refer to the second field you use:
Simple, all you have to remember is the ID, and call it directly using getElementById