You could just do
$USER1$/check_http -H $ARG1$ -p $ARG2$ $ARG3$
And set $ARG3$ if you need it.
Question
Is there a way to put optional arguments in centreon/nagios
A simple example. In Centreon:
$USER1$/check_http -H $ARG1$ -p $ARG2$ -w $ARG3$
Host (-H) and port (-p) arguments as required and warning argument (-w) set it as optional
Solution
You could just do
$USER1$/check_http -H $ARG1$ -p $ARG2$ $ARG3$
And set $ARG3$ if you need it.