For example you have such simple form:
form action="some-site.com/some-script.cgi" method=post
input type=text name=login value=''
input type=password name=password value=''
input type=submit name='Submit' value='Login'
User fill this form with value login: Peter and password: 1234 and click submit. If everything is Ok, script prints "LoginOk" on the result page.
For making a monitoring task to control this script, create task with next fields:
Http Method: choose POST
In "POST parameters" field you should add three strings:
In "Content check" field: LoginOk
Results: HostTracker with every check will fill this form and assume that it is OK if the set keyword LoginOk is returned.