Expected format array/collection

Hello,

I would like to update the metadata (custom field) of a user.
I want to map my result like that (with an array or a collection) :

{
  "RDV": {
    "RDV_{{4.id}}": {
      "Date_rdv": "{{4.date}}",
      "heure_debut": "{{4.time}}",
      "heure_fin": "{{4.endTime}}",
      "prestation": "{{4.type}},
      "options": "{{4.addonIDs}}",
      "prix_prestation": "{{4.price}}"
    }
  }
}

Can you give me the expected format? Because when I try with this format I get an error: Array of objects expected in parameter ‘meta’.

I make some test but always this error.

For your information, I can have several “RDV”.
All RDV must be in {…}, but each RDV own information must be in its own collection.

The end result would be, for example:

{
  "RDV": {
    "RDV_{{4.id}}": {
      "Date_rdv": "{{4.date}}",
      "heure_debut": "{{4.time}}",
      "heure_fin": "{{4.endTime}}",
      "prestation": "{{4.type}},
      "options": "{{4.addonIDs}}",
      "prix_prestation": "{{4.price}}"
    },
    "RDV_{{4.id}}": {
      "Date_rdv": "{{4.date}}",
      "heure_debut": "{{4.time}}",
      "heure_fin": "{{4.endTime}}",
      "prestation": "{{4.type}},
      "options": "{{4.addonIDs}}",
      "prix_prestation": "{{4.price}}"
    }
  }
}

Thank you !