After data verification, the sql query to populate a row in the Customers table goes something like this
mysqli_query($connect, "INSERT INTO Customers (CustFirst, CustLast, CustAddress)
VALUES ('$CustFirst', '$CustLast', '$CustAddress')")
mysqli_query($connect, "SELECT CustID from Customers
echo "The Customer " . $CustFirst . " " . $CustLast . " was added<br>";
echo "with " . $CustID . " being the number associated with the account.";
$CustID = mysqli_insert_id(connection);
You can write above code just after executing your insert query. This will return latest primary key id inserted in your database.