How to have a global variable which stores objects?

I have a class that is my fake repository, I created a global variable list<Ticket>, when I add some objects to the AddNewTicket() function, I want it to be added to addedTickets variable, and again when the AddNewTicket() function called, I want the previous values remain, and the new ticket would add at the end of the list. here is what I have tried, but the list would get empty for each request.

public class TicketRepository : ITicketRepository
{
    private readonly FakeDbContext _context;
    List<Ticket> addedTickets = new List<Ticket>();

    public TicketRepository()
    {
        _context = new FakeDbContext();
    }

    public bool AddNewTicket(Ticket ticket)
    {
        _context.Tickets.Add(ticket);
        addedTickets.Add(ticket);
        return true;
    }

 }

my program.cs

builder.Services.AddTransient<ITicketService, TicketService>();
builder.Services.AddTransient<ITicketRepository, TicketRepository>();

%%%%%%%%%% I want something like in-memory Database. %%%%%%%%%%