× Did you mean /sharing/rest/...?

You are using an unsupported version of this API located at /sharing/. Avoid using and/or developing with this unsupported version. There are pitfalls in doing so and no advantages over using the released and supported version of the API, which is logically equivalent. The supported version is located at /sharing/rest/.
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Sharing API Info

Owning System URL  
https://portal.nonprod.gcp.gis.kier.co.uk/arcgis
Token Service URL  
https://portal.nonprod.gcp.gis.kier.co.uk/arcgis/sharing/rest/generateToken