Trigger A Process Model That Has A Start Form From An Interface

I have a process model with three required parameters, A, B, and C. 

I have an interface that captures the values for inputs A and B, whereas C is captured in the Process Start Form of the process model.

I want to pass A, B from the interface into the process model, and also trigger the process start form so that I may pass the value for input C into the process model, all with the press of a button. 

Is there any way I could do this? a!startProcess and a!startProcessLink seem to bypass the process start form for some reason that I don't currently understand. 

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