POST /v1/functions/{functionId}/variables HTTP/1.1
Host: HOSTNAME
Content-Type: application/json
X-Appwrite-Response-Format: 1.0.0
X-Appwrite-Project: 5df5acd0d48c2
X-Appwrite-Key: 919c2d18fb5d4...a2ae413da83346ad2

{
  "key": "[KEY]",
  "value": "[VALUE]"
}