What to put in the "Newsletter form action" field ("i am using phplist")

First you have to create a subscription page in phplist. Then view that page and copy its address from the browser. This is the input for the newsletter form action field.

Putting “email” into the field for “E-mail identifier” will fill out the email field on the subscription page.

Check “Don’t display email confirmation” to disable the second field, so the user doesn’t have to fill that in.

Haven’t found a way yet to completely avoid being redirected to the phplist subscription page, so the user still has to press the “Subscribe to selected newsletters button” there to complete subscription. But otherwise it works fine.