Mendix is object based, so you cannot pass a direct value from a page. though you can pass the whole object of the context entity to the microflow using a object type input parameter.
This object contains the attribute used in the dropdown.
Accessing the value can be done by using this expression $ObjectVariableName/AttributeName
Incase you are working with the reference selector (rendered) as dropdown, then the same applies, but the expression is a bit different;