I like Integromat. It has some nice functions and you can restrict IP on webhooks.
But it has some strange things. It is not so obvious, but to connect Webhook to InvoiceNinja and to see what you can map you need to:
1. Initially you have to first re-determine data structure where you have to execute the trigger site (in my case sending the Formidable Contact form on WordPress)
2. Then you have to Run once and again execute the trigger site (in my case WordPress/Formidable)
3. Then checkmarking “Show advanced settings” in Integromat Action Invoiceninja Window
4. Mapping the Trigger fields (Formidable) to Action fields (Invoiceninja)