Es is a pronoun that can be used to signal that the subject of the sentence is in an unusual place (In German, in the "Mittel-" or "Nachfeld"), where it normally would only happen to end up in a question, for example) .
Ist eine Anforderung eingegangen? (I)
reverses "normal" German Word order (SPO - Subject, Predicate, Object) to PS, to signal a question
Eine Anforderung ist eingegangen (II)
is formed with standard word order (SP[O]), building a normal sentence
Es ist eine Anforderung eingegangen (III)
reverses standard word order like in (I), but to not sound like a question, something needs to be fixed, and that is done by placing an announcement of the subject in its normal place, formed by "Es". Es itself is in no way a subject of the sentence but simply a pronoun that signals (and allows) this exceptional word order.
Form (III) is normally used to place emphasis on the subject by putting it in an unusual place and signalling that fact through "Es". Famous example from fairy tales:
Es war einmal [eine Prinzessin | ein Frosch | ein König | ...]
Es can be used as a subject in impersonate sentences, like
Es regnet in Strömen
but obviously, here it is the single subject of the sentence.