Graphql federation - Communication between subgraphs for validation

Question

I am trying to implement Graphql federation with multiple subgraphs using Nestjs + Apollo GraphQL.

So I have USER subgraph and Application subgraph as separate Graphql services. Everything seems to be working fine when its comes to querying from the supergraph.

But I am not able to figure out how to implement the following

As Application subgraph have the following mutation to add the new application

type Mutation {
  createApplication(
    createApplicationInput: CreateApplicationInput!
  ): Application!
 
}

input CreateApplicationInput {
  name: String!
  userId: Int!
}

Now while calling the above mutation I am able to add new application, but not able to validate if user exists on user subgraph or not.

So my question is how I can validate that user exists before insert to application at graphql side