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General authorisation information can be found on the Authorization page in this guide. The endpoint however for the Sandbox server is:
https://apigo.actionsteplabsactionstepstaging.com/api/oauth/authorize
And requesting a token once authorised is:
https://api.actionstepactionstepstaging.com/api/oauth/token
Request Endpoint
The Sandbox endpoint for making resource requests with a valid token is:
https://api.actionsteplabsap-southeast-2.actionstepstaging.com/api/rest
Note that this endpoint is dynamic and is given to you as part of the Access Token. The above URL is for reference only.