Update process variable from webAPI call

How do I update a process variable in  process model instance via WebAPI ? 

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The use case is an external system which triggers an onboarding process. 

It calls a webAPI which starts an onboarding process. An email is being sent

to the customer for email verification. The process waits for the code to be entered

on the external system form. Once entered on the external system form, it should be 

posted to the running process instance  to resume execution.

So I need a way for the external system to post the code to the 

previously initiated process.