This is the code I used:

#include <stdio.h>
#include <string.h>

#define MAX_ACCOUNTS 255

typedef struct
{
    unsigned int id;
    char account_creation_date [10];
    char first_name [255];
    char last_name [255];
    char country_code [2];
    unsigned int iban;
    char password [255];
    double balance;
} account;

account accounts_db[MAX_ACCOUNTS];
unsigned int accounts_created = 0;

account get_account_id (unsigned int id)
{
    int i = 0;
    while(i < MAX_ACCOUNTS)
    {
        if(accounts_db[i].id == id)
        {
            return accounts_db[i];
        }
        i++;
    }
    account account;
    account.id = -1;
    return account;
}

void create_account(char first_name [255], char last_name [255], char password [255], char country_code [2])
{
    account new_account;
    new_account.id = accounts_created;
    strcpy(new_account.first_name, first_name);
    strcpy(new_account.last_name, last_name);
    strcpy(new_account.password, password);
    strcpy(new_account.country_code, country_code);
    strcpy(new_account.account_creation_date, "");
    new_account.balance = 0.0;
    new_account.iban = 0;
    accounts_db[accounts_created] = new_account;
    accounts_created++;
}

int main()
{
    char first_name [255]  = "Max";
    char last_name [255] = "Mustermann";
    char country_code [2] = "DE";
    char password [255]= "password";
    create_account(first_name, last_name, password,country_code);
    account account = get_account_id(0);
    printf("Name: %s %s \n", account.first_name, account.last_name);
    printf("Account creation date: %s\n", account.account_creation_date);
    printf("IBAN: %s %d", account.country_code, account.iban);
}```

When you run it you can see, that behind the country code of the IBAN you get the first two letters of the surename