To make the optional parameter work you can use the interpolation variable in the “post body” of the HTTP Client route by selecting the POST Request instead of GET request option. If you want to send a GET request you have to specify the values while creating the base view.
Whereas, when you select the POST request, VDP has the option of using interpolation function called “ExecuteIfIsNotNull” which you can use it in the post body.
You can also create a derived view to add a view parameter, for example view parameter called page_size_input with default value 100 and then add a "Where Condition" like "pageSize = page_size_input", so if you don't provide a value for "page_size_input" it will use the default value 100, but if you provide it, it will use the provided one as input.
To know more about view parameters and how the interpolation variables work read the sections”*Using the WHEREEXPRESSION variable*” and “*Parameters of Derived Views*” of the** Virtual DataPort Administration Guide.**
Hope this helps.