update a choice column in sharepoint

i want to create a clinic request in a company that would automate the approval of clinic requests.
i created a custom list on SharePoint called clinic Request with the following columns:
employee name – Single line of text
employee no. – Single line of text
Supervisor – choice column
.
.
When an employee submits a clinic request by create new list item to fill up the form request, a task needs to be assigned to their supervisor so the request can be approved or rejected.

in the first time he enters and wants to create new item he choose his supervisor from the choice column ,then when he wants to create another request the choice column will show the last choice of the employee.in order to prevent any mistakes could happen.
here i used the cookie to save the user first choice then i use the cookie another time to assign the value of the choice column when the employee wants to create a new item list .

i hope i described the scenario clearly

i am not using profile synchronization. i am using sharepoint designer to add java script code in the master page
my problem is in the right way to alter a field in sharepoint list .