How do I accept the configuration screen that pops up when installing Postfix?
This is similar to this question for accepting an agreement in a debconf question.
⇆ TAB , then return ↵ will let you continue.
Try this command in terminal: sudo dpkg-reconfigure postfix
This will bring up the configuration window again [where you left off]. Use the up/down arrow keys on your keyboard to select an option [where more than one option is displayed], and the left/right arrow key to choose [ok] so the next window option follows on etc. till all fields are completed by you