diff --git a/passkey/fastapi/admin.py b/passkey/fastapi/admin.py
index 3379aa5..62aca67 100644
--- a/passkey/fastapi/admin.py
+++ b/passkey/fastapi/admin.py
@@ -1,13 +1,13 @@
 import logging
 from uuid import UUID, uuid4
 
-from fastapi import Body, Cookie, FastAPI, HTTPException
+from fastapi import Body, Cookie, FastAPI, HTTPException, Request
 from fastapi.responses import FileResponse, JSONResponse
 
 from ..authsession import expires
 from ..globals import db
 from ..globals import passkey as global_passkey
-from ..util import frontend, passphrase, permutil, querysafe, tokens
+from ..util import frontend, hostutil, passphrase, permutil, querysafe, tokens
 from . import authz
 
 app = FastAPI()
@@ -335,7 +335,7 @@ async def admin_update_user_role(
 
 @app.post("/orgs/{org_uuid}/users/{user_uuid}/create-link")
 async def admin_create_user_registration_link(
-    org_uuid: UUID, user_uuid: UUID, auth=Cookie(None)
+    org_uuid: UUID, user_uuid: UUID, request: Request, auth=Cookie(None)
 ):
     try:
         user_org, _role_name = await db.instance.get_user_organization(user_uuid)
@@ -358,7 +358,9 @@ async def admin_create_user_registration_link(
         expires=expires(),
         info={"type": "device addition", "created_by_admin": True},
     )
-    origin = global_passkey.instance.origin
+    origin = hostutil.effective_origin(
+        request.url.scheme, request.headers.get("host"), global_passkey.instance.rp_id
+    )
     url = f"{origin}/auth/{token}"
     return {"url": url, "expires": expires().isoformat()}
 
diff --git a/passkey/fastapi/api.py b/passkey/fastapi/api.py
index a247e03..6b08906 100644
--- a/passkey/fastapi/api.py
+++ b/passkey/fastapi/api.py
@@ -29,7 +29,7 @@ from ..authsession import (
 )
 from ..globals import db
 from ..globals import passkey as global_passkey
-from ..util import passphrase, permutil, tokens
+from ..util import hostutil, passphrase, permutil, tokens
 from ..util.tokens import session_key
 from . import authz, session
 
@@ -267,7 +267,9 @@ async def api_create_link(request: Request, auth=Cookie(None)):
         expires=expires(),
         info=session.infodict(request, "device addition"),
     )
-    origin = global_passkey.instance.origin.rstrip("/")
+    origin = hostutil.effective_origin(
+        request.url.scheme, request.headers.get("host"), global_passkey.instance.rp_id
+    )
     url = f"{origin}/auth/{token}"
     return {
         "message": "Registration link generated successfully",
diff --git a/passkey/util/hostutil.py b/passkey/util/hostutil.py
new file mode 100644
index 0000000..d6551e0
--- /dev/null
+++ b/passkey/util/hostutil.py
@@ -0,0 +1,24 @@
+"""Utilities for host validation and origin determination."""
+
+from ..globals import passkey as global_passkey
+
+
+def effective_origin(scheme: str, host: str | None, rp_id: str) -> str:
+    """Determine the effective origin for a request.
+
+    Uses the provided host if it's compatible with the relying party ID,
+    otherwise falls back to the configured origin.
+
+    Args:
+        scheme: The URL scheme (e.g. "https")
+        host: The host header value (e.g. "example.com" or "sub.example.com:8080")
+        rp_id: The relying party ID (e.g. "example.com")
+
+    Returns:
+        The effective origin URL to use
+    """
+    if host:
+        hostname = host.split(":")[0]  # Remove port if present
+        if hostname == rp_id or hostname.endswith(f".{rp_id}"):
+            return f"{scheme}://{host}"
+    return global_passkey.instance.origin