When creating a custom button, I see an $Action value I can't find documentation on - $Action.Custom_Object__c.CreateFromLookup. It's not on the list of Valid Values for the $Action Global Variable.
It only seems to work for buttons on Other_Obj__c for Custom_Object__c where Other_Obj__c has a lookup to Custom_Object__c.
The closest I can find is the poorly documented part of the UI API at https://developer.salesforce.com/docs/atlas.en-us.uiapi.meta/uiapi/ui_api_resources_lookup.htm#stq=createfromlookup&stp=1
The result of {!URLFOR($Action.Custom_Object__c.CreateFromLookup)} on a Other_Obj__c page button seems to send me to the Custom_Object__c tab.
