Android KeyStore with StrongBox: How to test for User Presence?

Android API 28 running on a Pixel 3 gives the option to require User Presence for keys generated inside the Android KeyStore. But how do I actually test for user presence when creating a signature with that key? Am I missing something in the docs?

KeyGenParameterSpec.Builder keyGenSpec = new KeyGenParameterSpec.Builder("alias", KeyProperties.PURPOSE_SIGN)
    .setDigests(KeyProperties.DIGEST_SHA256)
    .setKeySize(256)
    .setIsStrongBoxBacked(true)
    .setUserPresenceRequired(true);

KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("EC", "AndroidKeyStore");
keyPairGenerator.initialize(keyGenSpec.build());
keyPairGenerator.generateKeyPair();

KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
keyStore.load(null, null);
PrivateKey privateKey = (PrivateKey) keyStore.getKey("alias", null);
Signature signature = Signature.getInstance("SHA256withECDSA");
signature.initSign(privateKey);
signature.update("Hello".getBytes(Charset.defaultCharset()));
byte[] sign = signature.sign();

That code throws android.security.KeyStoreException: -69 which translates to PROOF_OF_PRESENCE_REQUIRED. I've also tried to wrap the signature process in an BiometricPrompt, but to no avail.