Select an already existing ArcGIS Server account (or create a new one) and insert the missing part of the service URL from the ArcGIS Server Manager. You can find this under Services by clicking on the desired Map Service. At Functions you will find under the subitem URLs the REST URL. Copy here the part of the URL to <server>:<port>/arcgis/rest/services/.

 

ArcGIS Server Manager: Find REST URL

 

Create New Project: Main Map Service

 

Click Next.