Getting VisitorId from UserID

I want to embedd the the Visitor Profile in another application where I have the user-id.

Unfortunately the link provided only uses the visitor-id which my backend system does not know: /index.php?date=2017-02-08&module=Widgetize&action=iframe&visitorId=1da16d53ff199a89&idSite=1&period=day&moduleToWidgetize=Live&actionToWidgetize=getVisitorProfilePopup

is there another link where I could use the User-ID or is there an API Method to get the visitor-Id for a given user-id?