diff --git a/18.md b/18.md
index fb6030f..dc76400 100644
--- a/18.md
+++ b/18.md
@@ -28,6 +28,8 @@ If `SERVICE` wants to get one or more types of payer identities from `WALLET` th
 +       "mandatory": boolean,
 +       "k1": string // hex encoded 32 bytes of challenge
 +    },
++    "contentMetadata": { "mandatory": boolean },
++    "contentMetadataUri": { "mandatory": boolean },
 +    ...other fields may be negotiated
 +  },
    "tag": "payRequest",