How to make a handler for information sent via https://vk.com/dev/apps.sendRequest to another device?

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Good evening. There is a mobile game on Unity in which a player logs in through a contact and sends information to another authorized player through https://vk.com/dev/apps.sendRequest . How can the other player get this information, where does it come from? I'm trying to figure out how to make a handler for this information in Unity (a specific situation - one player sends another an invitation to a PvP battle, the second player opens a window in which he sees information about the proposed PvP battle).