Hi Geurt,
It looks to be that you want to supply the query string "sinceDatetime" with a value from the parameter {current_date}.
To do this, current_date has to exist as an Input parameter of this endpoint. This way, the input parameter can be supplied a value in the Web connection process action input parameters, either as a constant value or a variable determined by logic for example.
Do note the format the API requires. You will need to format the date so the API accepts it.
Hope this helps!