My fiance recently went through the process of the K1 visa at Islamabad embassy. For him, he did not get any confirmation/ letter from the embassy. From what I understand if your status on the CEAC website says "Ready" you can go ahead and submit the DS-160. Once you have submitted your DS-160 and you have paid your visa fee then you can go to ustraveldocs to schedule your interview and then you can also schedule your medical. You will need the confirmation of submitting the DS-160 and also your interview confirmation but you don't need anything specific to be sent from the embassy. We ended up waiting a little over a month to get something from the embassy and when we didn't get anything I sent an inquiry which simply confirmed that our case was at the embassy. My fiance probably could have submitted the DS-160 a few weeks sooner.
Best of luck with the rest of the K1 visa process!
