Hi Jochen, I believe you should be able to do this by using one of the following OOTB steps: 

1. Products -> Variable State Validation: This step can be used to check the state of the variable such as (mandatory, visible, read-only, etc..)
2. Products -> Validate Variable Values: This step can be used to validate the actual value that was selected within that variable.
3. Products -> Set Variable Values: This step will populate a variable with a value. 

You have to make sure the task/form is first opened, then you can validate the variables. I hope this helps.

Thanks

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