Representation always specifies data sent in body of HTTP message, and content-type specifies it's format. Given that Azure API Management service does no processing on representation sample you specify you should always set sample to correspond to the content type you've specified.
In the example above you've used application/x-www-form-urlencoded, so more appropriate form of representation sample would be:
[email protected]&password=123456
Such operations should use POST verb and sends data in message body.
If you want to specify operation that accepts data in query string, like
http://some.domain/user/[email protected]&password=123456
you should use Parameters section and a GET verb. If you want to allow both - you have to create two operations.