title: Pass parameters to Sign-up policy
body: <p>I am creating B2C users with Social Identity Providers with the help of a Sign-up policy but we have a requirement to add some user attributes (extended properties) to this new user. For example set "AccountId" for the user. </p>
  
  <p>If i add "AccountId" as a Sign-up Attribute and enter some value it works fine, when I check the user properties via Graph API the "AccountId" is correct.</p>
  
  <p><a href="https://i.stack.imgur.com/2B9hm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2B9hm.png" alt="enter image description here"></a> 
  <a href="https://i.stack.imgur.com/ok385.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ok385.png" alt="enter image description here"></a></p>
  
  <p>But in this case the "AccountId" should not be editable or visible to the user, I just want the Sign-up policy to add "AccountId" to the user created with for example facebook,as a hidden field on the Sign-up page.</p>
  
  <p>Is it possible, from my ASP.Net MVC application using Azure B2C AD, to pass this value to the Sign-up page and associate it with a Sign up attribute ? Can it be done via parameter (&amp;accountid=1234) or from some OpenId-propperties  ?</p>