I found it myself by checking the shape field when filling in a certain country when using the Esri maps Locate Workflow.
You have to fill in something like this:
{"spatialReference":{"wkid":4326},"rings":[[[5.4739268537583,49.5006177334594],[5.42816555029115,49.5958898607962],[5.34416857964955,49.6376036740587],[5.31342149326235,49.6108248562936],[5.33277008158818,49.6559716805107],[5.26730086368155,49.6983719927301],[...]]]}
But I saw I can't create shapes who have more than 255 characters...
So there is still a problem for shapes who are more complicated.
Regards,
Sara