How can I let the user choose the target mailing list on my web form?
What you can do is to create a combo box on the form that lets you choose the target mailing list and then use the "Conditions" button on the "Opt-in" tabs of your mailing lists to check for the presence of the given value in the body of the email sent by the form ("The body contains specific words").
Note: You can add as many <option> tags as you like.
Let's use the above code as an example. You need two mailing lists to implement the steps below.
Mailing list #1:
Double-click the list icon.
Click the "Opt-in" tab.
Click the "Conditions" button.
Click "Incoming messages that match...".
Turn on "The body contains specific words", and click "words".
Add the following -- target: list1
Mailing list #2: Repeat the steps above but use the following in the last step -- target: list2
IMPORTANT: I forgot to mention that both these lists must download mail from the same email account -- it must be the account the form sends submitted information to.