"Standard" workflows are dedicated to server-side execution, not for client-side execution. So you can't manage an input form in it.
Instead, you can achieve these alternative ways below, depending on your use case.
If you need an UI based on Adobe Campaign windows client (it means that the user has an operator account), you can carry out a standard form that only requires XML syntax to define the form. Typically it is used for wizard form interaction such new delivery wizard, new webApp wizard, etc. The input fields typed by the user can be stored in variables (/tmp xpath) or in schemas/tables such as xtk:options or whatsoever (for example a custom table for storing parameters or other data).