clients... then MUST make an HTTP POST request to this URL with the Content-Type of application/ld+json; profile="https://www.w3.org/ns/activitystreams".

Requirement

clients... then MUST make an HTTP POST request to this URL with the Content-Type of application/ld+json; profile="https://www.w3.org/ns/activitystreams".

Origin

The origin of this requirement is a quote from the ActivityPub specfiication:
prefixTo do this, clients MUST discover the URL of the actor's outbox from their profile
and then MUST make an HTTP POST request to this URL with the Content-Type of application/ld+json; profile="https://www.w3.org/ns/activitystreams".
suffix Servers MAY interpret a Content-Type or Accept header of application/activity+json as equivalent to application/ld+json; profile="https://www.w3.org/ns/activitystreams" for client-to-server interactions.
https://www.w3.org/TR/activitypub/ , § 6

id

urn:uuid:222934ff-e0e2-4b3b-a0d8-72908d1698b1

uuid

222934ff-e0e2-4b3b-a0d8-72908d1698b1

Full JSON

{
  "id": "urn:uuid:222934ff-e0e2-4b3b-a0d8-72908d1698b1",
  "type": "Behavior",
  "uuid": "222934ff-e0e2-4b3b-a0d8-72908d1698b1",
  "content": "clients... then MUST make an HTTP POST request to this URL with the Content-Type of application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\".\n",
  "tag": [
    {
      "name": "ActivityPubClient",
      "id": "https://socialweb.coop/tag/ActivityPubClient"
    }
  ],
  "context": [
    "https://www.w3.org/TR/activitypub/",
    {
      "name": "Client to server interaction"
    },
    {
      "name": "clients posting Activities to an actor's outbox"
    }
  ],
  "origin": {
    "source": "https://www.w3.org/TR/activitypub/",
    "section": {
      "id": "https://www.w3.org/TR/activitypub/#x6-client-to-server-interactions",
      "branch": [
        6
      ]
    },
    "selector": {
      "type": "TextQuoteSelector",
      "prefix": "To do this, clients MUST discover the URL of the actor's outbox from their profile\n",
      "exact": "and then MUST make an HTTP POST request to this URL with the Content-Type of application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\".\n",
      "suffix": "Servers MAY interpret a Content-Type or Accept header of application/activity+json as equivalent to application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\" for client-to-server interactions. \n"
    }
  },
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}

Links