You must know that the powershell cmdlet Add-WfaWorkflowParameter is used to save a dynamically obtained value during command execution. It can't be used in planning.
A constant in the workflow tab is going to be evaluated during planning/preview when no commands have actually started to execute, only the input parameters to the commands have been resolved. So at this time there is nothing that has been added by Add-WfaWorkflowParameter at all.
But why you need this? This looks like s solution being attempted for a requirement. If you can elaborate this, I can suggest something.
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