How to upload data into Vitruvi using APIs

Hi discussion team, we at Realworld are currently working on a project with Vitruvi for a UK Alt Net. We are currently providing data from the planning /inventory solution in a set of JSON files which are being manually uploaded into Vitruvi before your system is used to generate the work instructions. We have now been asked to upload the data using APIs. We had a discussion with somle members of your team last week, and the APIs that they recommended we used to pass the data are:
Create Design Element – we understand that this will create a new instance of one of the inventory features which we currently output in json – e.g. a pole or a duct
We were then advised that we would need to add the values for the custom fields linked to this object – not quite sure which API we should use for this.
Can you confirm if this is the best approach, or if we are able to e.g. upload an entire feature/custom element from a json file