How to get forms element value from onSubmit event?

First you have to create a form element and put your input or whatever in a form-item. Give a name to your form through the properties panel (“formSample” in the screen below) and give a “control name” to your input the same way (here “myFormItem”)

You’ll be able to pick the value from source picker by clicking the form icon (the 6th one) see below :