General advice / guidelines on how to implement MVC

Question

HI all,
 i have recently moved from ASP.Net WebForms to ASP.Net MVC and having trouble with deciding on how to move forward with a particular implementation of a Modal and GUI design.
This is the Model:


I want to design a Create page on which the User details can be entered, as well as a list (bootstrap-table) of User_Customers for that User.



When clicking on the green + button at the right-bottom, I would like to show a bootstrap modal window in which a User_Customer can be added.



I have thought of the following 2 options:

1. Add the User_Customer on the client with jQuery. Use proper naming and indexes so that the list is received on the server when the User hits the "Create' button.
   
2. Perform an intermediate postback and add the User_Customer on the server. Not sure what to add the User_Customer to, as the User record does not exist yet until the complete form is posted back.
   

What is the best way to move forward and why?