Intercept variable changes?

Question

PHP

There is a task - now when a user has registered and did not specify a first and last name, then the Default Name is assigned to him, I need to implement that when the user makes a purchase (where he enters his data), the first and last names are transferred to him in the profile. The form is made through Vookumers. Example in the picture:

Answer 1

When making a purchase, parse the first and last name with JavaScript and store them in localStorage.
Further, if the user suddenly wants to register, pull out the information from memory and substitute it in the form.