curl --request POST \
  --url https://api.mnemom.ai/v1/billing/checkout \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "plan_id": "<string>",
  "annual": false,
  "promo_code": "<string>"
}
'

{
  "url": "<string>",
  "session_id": "<string>"
}