Generate onboarding URL

curl --request POST \
  --url https://api.etherfuse.com/ramp/onboarding-url \
  --header 'Content-Type: application/json' \
  --header 'X-API-Key: <api-key>' \
  --data '{
  "customer_id": "123e4567-e89b-12d3-a456-426614174000",
  "bank_account_id": "123e4567-e89b-12d3-a456-426614174001",
  "public_key": "9Qx7r...",
  "blockchain": "solana"
}'

{
  "presigned_url": "<string>"
}