asp.net mvc - Set value for joined table in MVC Razor -

i'm trying create razor page windows auth mvc app tracks created record. basic idea looks this:

public class user {     public int id {get;set;}     public string networkid {get;set;} }  public class event {     public int id {get;set;}     public int userid {get;set;}      public datetime created {get;set;}      public virtual user user {get;set;} } 

on razor page, i'm trying create hiddenfor set user id , can't post correctly. setting created field works correctly (@html.hiddenfor(model => model.created, new { value = datetime.now })), userid won't set.

this doesn't work:

@html.hiddenfor(model => model.user.networkid, new { value = user.identity.name }) 

this throws null reference exception:

    @{         model.user.networkid = user.identity.name;     }     @html.hiddenfor(model => model.user.networkid) 

there has easier way this, other looking networkid on table , pulling actual primary key.

any ideas?