Apollo Federation: How to access data from Input Request across federated services through resolver

Question

Looking for an option to join graphQL API response using Apollo federation where Graphql APIs are built on top of POST services.

To explain my usecase, I have created below dummy example.

Two POST Services are

1. User Service and
2. Post Service

Request for UserService

{
userid : "13242"
}

Response from UserService

    {
       userName,
       email,
       dateOfBirth,
       postIds []
    }

Request for PostService

  {
    postId: "13242",  // this is from UserService Response
    memberInfo: {     // this will come from client
       memberid : "34343",
       memberGroup : "mg-8798",
       memberLocation : "LOc-87"
    },
    groupInfo: {     // this will come from client
       GroupId :  "87980"
       GroupInfo : "some group Info"
    }
  }

Post Service needs PostIds from UserService Response and memberInfo and GroupInfo from Gateway Input (Input from Client).

In my current resolver I don't have any information other than postId,

{
  user: {
    posts(user) {
      return user.posts.map(postId => ({ __typename: "Post", postId }));
    }
  }
}

How do I write resolver where two responses need to get connected using postId but need extra info from client Input?

or In other words - How to access data from Input Request across federated services through resolver

---

Added to answer to one of the Questions in comments

---------------------------- User Schema ---------------------------

 type User @key(fields: "id"){
    id: ID!
    name: String!
    username: String!
    userposts: [Post]
    email: String!
    phone: String!
    website: String!
  }

// Here Id (postId) should be primary key, but service needs other inputs too which contains MemberInfo and GroupInfo. I am not sure how to do this and this is where I need help as well as on resolver

 extend type Post @key(fields: "postInput") {
      id: ID! @external
      postInput : PostInput
  }

-----------------------------Post Schema -------------------------

 type PostResult @key(fields: "postInput"){
    postInput: PostInput!
    userid: Int!
    title: String!
    comments : [Comments]
  }  

input PostInput {
   postId : String!
   memberInfo: MemberInfo 
   groupInfo: GroupInfo
}

input MemberInfo { 
   memberNumber: "String"
   enrolmentId: "String" 
}

input GroupInfo {
    groupIds: "String"
    groupName: "String" 
}