This is specific to Vietnam. I do not know about other Consulates.
Create USTravelDocs Account and Register Delivery Address
After receiving the notification letter from the National Visa Center (NVC) which tells you that the NVC will forward the petition to our office, please immediately follow the step-by-step instructions below to create your USTravelDocs Account and register your delivery address. The Consulate needs this information in order to deliver the Instruction Package Letter (PKT3-K) as well as other documents to you.
As part of this process, you will be asked to provide your valid DS-160 barcode number. Therefore, please make sure you visit: https://ceac.state.gov/genniv/ and follow the directions on the website to complete your Form DS-160. After having submitted the DS-160 successfully, please print out the DS-160 confirmation page, which includes the DS-160 barcode number, for future use.
Step 1 – Create an account at: http://bit.ly/visa-account. If you already have an account, move to Step 2.
Step 2 – Log in to your account at: http://bit.ly/us-login.
Step 3 – After logging in, you will be taken to a Dashboard, please look at the left of the Dashboard, click on “New Application / Schedule Appointment”, and follow the instructions below:
Choose “Immigrant Visa”, then click “Continue”.
Choose “K1 Visas – Appointment Scheduling/Rescheduling and Fee payment”, then click “Continue”.
Select “K1 – Fiance or Fiancee of U.S. Citizen”, then click “Continue”.
Fill in all the required fields, then click “Continue”. Please make sure the First Name field includes both your first name and given name. For example, if your full name is Nguyen Thi Thu Trang, please enter “Thi Thu Trang” in the First Name field and “Nguyen” in the Last Name field. Please also make sure you enter your most updated Passport Number and please do not forget to provide your phone numbers. These fields are very important.
If you have unmarried children under 21 years old and wish for them to immigrate with you, you will be able to add them as dependents, then click “Continue”.
Select your preferred delivery option, then click “Continue”.
YOU HAVE NOW COMPLETED REGISTERING YOUR DELIVERY ADDRESS. Please STOP HERE! Log out of the system and do not take any further action. Your Instruction Package Letter (PKT3–K) will be delivered to you at the registered address. You can continue further steps after receiving the Instruction Package Letter (PKT3–K) from the Consulate