`hydra -l username -P passwordlist  127.0.0.1 https-post-form "/index.php:UserName=^USER^&Password=^PASS^&submit=logout:invalid" -vV -f`

The option field (following the service field) takes three ":" separated
values and an optional fourth value, the first is the page on the server
to GET or POST to, the second is the POST/GET variables (taken from either
the browser, or a proxy such as PAROS) with the varying usernames and passwords
in the "^USER^" and "^PASS^" placeholders, the third is the string that it
checks for an *invalid* or *valid* login - any exception to this is counted
as a success.
So please:
 * invalid condition login should be preceded by "F="
 * valid condition login should be preceded by "S=".
By default, if no header is found the condition is assume to be a fail,
so checking for *invalid* login.