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The previous guide showed how to integrate IdentityServer with Authsignal in order to add MFA after a traditional username & password sign-in. In this guide, we’ll demonstrate how to let the user create a passkey and use it instead of their password the next time they sign in.

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Configuring passkeys

In the Authsignal Portal we have configured localhost as the Relying Party for the purposes of local development. We also whitelist both the login server and the application server as expected origins.
This means the user can enroll a passkey directly from our web app running on https://localhost:5002, then reauthenticate with the same passkey when signing in to IdentityServer running on https://localhost:5001.

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Enrolling a passkey

Once the user has logged in with MFA, we can use the Authsignal Web SDK to allow the user to add a passkey. This can be done by adding a button to the application server (i.e. the WebClient).
To implement this we add a new page /src/WebClient/Pages/Passkeys.cshtml. 
@page
@model PasskeysModel

@Html.HiddenFor(m => m.enrollmentToken)

<button id="add-passkey" onClick="addPasskey()">Add a passkey</button>

<script src="https://unpkg.com/@@authsignal/browser@@0.3.0/dist/index.min.js"></script>
<script src="~/js/passkeys.js"></script>
The enrollmentToken here is fetched server-side in /src/WebClient/Pages/Passkeys.cshtml.cs when the page loads. 
public async Task<IActionResult> OnGet()
{
  var trackRequest = new TrackRequest(
    UserId: User.Claims.First(x => x.Type == "sub").Value!,
    Action: "add-passkey",
    Attributes: new TrackAttributes(
      Scope: "add:authenticators"
    )
  );

  var trackResponse = await _authsignal.Track(trackRequest);

  this.enrollmentToken = trackResponse.Token;

  return Page();
}
Finally, we need to add the client-side JS implementation for the addPasskey function which uses the Authsignal Web SDK. 
function addPasskey() {
  var client = new window.authsignal.Authsignal({
    tenantId: "YOUR_TENANT_ID",
    baseUrl: "https://api.authsignal.com/v1", // Update for your region
  });

  var token = document.getElementById("enrollmentToken").value;

  client.passkey.signUp({ token }).then((resultToken) => {
    if (resultToken) {
      alert("Passkey added");
    }
  });
}

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Signing in with a passkey

Now that the user has enrolled a passkey, there are two ways to offer passkey sign-in on the IdentityServer login page.

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Passkey autofill

Passkey autofill is an unobtrusive approach where the user’s available passkeys are suggested automatically when they focus the username field. First we need to ensure that the input field has the value username webauthn in the autocomplete attribute. 
<input
  id="passkey-username"
  class="form-control"
  placeholder="Username"
  asp-for="Input.Username"
  autocomplete="username webauthn"
  autofocus
/>
Then we add some JavaScript to run when the page loads. 
function initPasskeyAutofill() {
  var client = new window.authsignal.Authsignal({
    tenantId: "YOUR_TENANT_ID",
    baseUrl: "https://api.authsignal.com/v1", // Update for your region
  });

  client.passkey.signIn({ autofill: true }).then((token) => {
    if (token) {
      var returnUrl = document.getElementById("Input_ReturnUrl").value;

      window.location = `https://localhost:5001/Account/Login/Callback?returnUrl=${returnUrl}&token=${token}`;
    }
  });
}

if (document.readyState === "loading") {
  document.addEventListener("DOMContentLoaded", initPasskeyAutofill);
} else {
  initPasskeyAutofill();
}
When a user focuses the input field and selects their passkey, the method resolves with a token.

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Sign-in button

A dedicated sign-in button launches the passkey prompt immediately when clicked, making passkeys a clearly visible option on your login page. Add a button to your IdentityServer login page: 
<button onclick="signInWithPasskey()">Sign in with passkey</button>
Then add the click handler, calling signIn() without the autofill flag: 
function signInWithPasskey() {
  var client = new window.authsignal.Authsignal({
    tenantId: "YOUR_TENANT_ID",
    baseUrl: "https://api.authsignal.com/v1", // Update for your region
  });

  client.passkey.signIn().then((token) => {
    if (token) {
      var returnUrl = document.getElementById("Input_ReturnUrl").value;

      window.location = `https://localhost:5001/Account/Login/Callback?returnUrl=${returnUrl}&token=${token}`;
    }
  });
}
In both cases, the validation process is the same as for signing in with MFA in the previous guide. The token returned from the Authsignal Web SDK is passed to the callback page, which validates the result of the challenge server-side and logs the user in.