iDRAC 8 does not support performing reset to default of iDRAC using redfish. You can use racadm to perform reset to default of iDRAC in iDRAC 8. 
Hi  according to the manual iDRAC8 with Lifecycle Controller Version 2.70.70.70 ,the iDrac 8 with firmware 2.75.75.0 should support following HTTP method Supported Action — ResetToDefaultsDescriptionThis defines the name of the OEM action supported when used in conjunction with a POST operation to this resource. When issued, this operation resets the Manager attributes to default settings./redfish/v1/Managers/ /Actions/Oem/DellManager.ResetToDefaults HTTP methods and privileges : POST Supported properties and values for ResetToDefaults Property Values ResetType • All • ResetAllWithRootDefaults • Default  Can anyone confirm that this works as im getting message :  
Redfish API reset idrac cfg to default on iDrac8