How to create an entity instance programmatically

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Does anyone know how to create an entity instance programmatically?  Based on selection on a form I need instances of and entity created, the data of which will come from an AJAX call.

 

I can see we can do CustomEntityCollect controls (see previous post) but this is not exactly what I want to do.  I just want to know the syntax for instance.add method, or whatever it is.

 

I can see drilling into the interview object that there is a method called generateEntityInstance but there is not documentation anywhere on this, or any other way to add an entity instance via code.  It must be able to be done surely?