post https://{environment}.masspay.io/{VERSION}/payout/account/topup
This POST endpoint is used to initiate a topup from the funding instrument to your MassPay account balance.
You can use this endpoint to start the topup process.
The body of the request requires the token of the funding instrument, the type of rail you would like to use, the amount to topup and the currency.
The response will include a JSON objects containing a success boolean.