docs: state the CLI access prerequisite for identity link web --app

docs/guides/managing-identities.md

@@ -144,7 +144,23 @@ icp identity import my-identity --read-seed-phrase
 
 Sign in to a web-based authentication service and link the resulting identity into icp-cli. **Internet Identity** is one example; the icp-cli default points at [id.ai](https://id.ai), but any service that publishes a `/.well-known/cli-auth-config` for the icp-cli web-auth protocol works the same way.
 
-With Internet Identity, every app sees a *different* principal for the same user. Linking a web-based identity means picking which app you want to sign in *as*. Pass that app's domain with `--app`:
+The simplest form gives icp-cli its own account with the auth provider (id.ai uses `cli.id.ai`), and works without any further setup:
+
+```bash
+icp identity link web my-cli-identity
+```
+
+This prints the sign-in URL and waits for you to press Enter, then opens your browser at the auth provider's sign-in page. Once you complete sign-in, the browser hands the delegation back to icp-cli and the identity is stored locally under the chosen name.
+
+### Signing In As a Specific App
+
+With Internet Identity, every app sees a *different* principal for the same user. To get the same principal an app sees in its own UI — your NNS neurons, your OISY wallet — pass that app's domain with `--app`.
+
+This requires one prerequisite: **you must enable CLI access for your Internet Identity first.** Internet Identity will not hand a command-line tool a delegation for another app's domain unless you have. Until you do, the sign-in page shows *"CLI access not enabled"* and the command never receives an identity.
+
+To enable it, sign in at [id.ai](https://id.ai) (Internet Identity), open your identity settings, and turn on **CLI access**. The setting is stored per browser and per identity, so enable it again on any other device or browser you sign in from.
+
+Then link the identity:
 
 ```bash
 # Sign in as your NNS identity
@@ -154,13 +170,7 @@ icp identity link web nns-identity --app nns.ic0.app
 icp identity link web oisy-identity --app oisy.com
 ```
 
-This opens your browser at the auth provider's sign-in page. Once you complete sign-in, the browser hands the delegation back to icp-cli and the identity is stored locally under the chosen name. The resulting principal matches the one the app would see in its own UI. Note that apps may use `alternativeOrigin` with II - the correct domain is `nns.ic0.app`, not `nns.internetcomputer.org`, which would give you a different principal.
-
-When `--app` is omitted, the auth provider picks its own default (id.ai uses `cli.id.ai`):
-
-```bash
-icp identity link web my-cli-identity
-```
+Some apps use II's `alternativeOrigin` mechanism, so the domain Internet Identity knows them by is not the one in your address bar — passing the wrong one links a different principal. `--app` needs the II origin. NNS is the common case: its II origin is `nns.ic0.app`, and icp-cli maps `nns.internetcomputer.org` onto it for you, so either spelling links the same identity.
 
 ### Using a Different Auth Provider