I would refer you to the USNA Admissions website, Select Apply (under the top menu bar), Select "Steps for Admission" from the drop down menu, then go to #3, Apply for Nomination. The direct website link is https://www.usna.edu/Admissions/Apply/index.php#fndtn-panel3-Steps-for
When on the Apply for Nomination page you can then find the JROTC nomination application in two different spots.
1. Under "Nomination Applications" select the item titled "ROTC/JROTC Unit Nomination
" and that will take you to the
pdf form that you need to provide to your JROTC instructor for completion and follow the steps for the form to be completed and returned to
Admissions. The form will be printed and mailed to the USNA Admissions office (follow the instructions on the form). There is not an online portal option as you mentioned that USAFA uses.
2. For a more in-depth amount of info as to the JROTC nomination on that same page as above "3. Apply for Nomination";
go to "Nomination Sources" and select "Reserve Officers Training Corps Units"
This brings you to a page that explains a little more about the nomination and it also contains the same pdf application form.
I hope the above is helpful to you and provides you further clarification about this nomination source. As always for all other applicants or interested parties, the USNA Admissions website has a wealth of information and the Steps for Admission is a first place to look for primary source information as to what guidance has been given from USNA as to the required steps for completing an application.