Does anybody here know how i can send a doc Id in a push notification without it being sen by user?

the firebase docs are entirely horribly written and confusing, and i was hoping someone here can help. In my cloud function, i have this as the payload for the notfi:

var message = {
notification: {
title: ‘Ruff, RrrrRuf! Gettin some action!’,
body: title
},

    android: {   
        notification: {   
          imageUrl: content  
        } 
      },
       apns: {  
        payload: {   
          aps: {    
            'mutable-content': 1   
          }  
        },
          fcm_options: {   
          image: content  
        } 
      },
    token: fcmToken,
}

It works. But when i try to add a “data” feild, like this, it does not work, and i get an error message in logs “unkown filed :data”

var message = {
notification: {
title: ‘Ruff, RrrrRuf! Gettin some action!’,
body: title
data: postId
},

    android: {   
        notification: {   
          imageUrl: content  
        } 
      },
       apns: {  
        payload: {   
          aps: {    
            'mutable-content': 1   
          }  
        },
          fcm_options: {   
          image: content  
        } 
      },
    token: fcmToken,
}