You could store it as variables
$account_id = 'your_account_id';
$password = 'your password';
someFunction($account_id, $password);
Or store it in {$table_prefix}_options
table and retrieve it by get_option
function
$account_id = get_option('account_id');
$password = get_option('password');
someFunction($account_id, $password);
You could save this values, for example from admin panel, by update_option
function.
Second option is better, because you can share your functions.php
with others and they will not know your credentials. It is also important, that password shouldn’t be stored as plain text.