Import donations from an external source using Form processor
We use the Form Processor at a few of our clients for scenario' s like yours. In most cases we use this in a configuration with CiviCRM on another server and outside traffic (including the Form Processor API request) coming in through CiviProxy (https://docs.civicrm.org/civiproxy/en/latest/). 

For this we use the CiviMRF framework (https://github.com/CiviMRF/cmrf_core).