How to get form property input data

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Hi, I am currently creating a project using flowable. in my application, I want to let the users to input data to “form property” from property panel, and the engine will then send that data to the next service task. However, there is only formProperty.getValue(), getId(), getName(). But on the form property panel, there is not a “value” option, the only things I can input data are “Expression” and " Variable". so my question is how can i get the “Expression” or “Variable” data?