Structures are a special case of scalar parameters. In case of structures, an individual variable doesn’t have to be assigned to the whole structure, but rather, to each element of the structure.
Please use the Edit link to open up an additional dialogue for assigning structure elements as shown in the screenshot.
Similar to pure scalar parameters it also applies to structure elements: Imports can contain constants or variables, exports are only allowed to contain variables. Of course all value fields could be left empty if the parameters are not needed.