Override.cmp

// cmp
<aura:component access="global" implements="lightning:actionOverride,force:hasRecordId,force:hasSObjectName,lightning:isUrlAddressable"
                description="proof of concept cmp">
    <aura:attribute name="contextId" type="String"/> 
    <aura:attribute name="windowURL" type="String"/> 
    <aura:handler name="init" value="this" action="{!c.init}"/>
    <lightning:card title="Details">
        <div class="slds-p-around_medium">

            <p>RecordId (should be empty for new action): <b>{!v.recordId}</b></p>
            <p>Name of object to be created or edited: <b>{!v.sObjectName}</b></p>
            <p>sObject from which action was invoked: <b>{!v.contextId}</b></p>
            <p>Current User Id: <b>{!$SObjectType.CurrentUser.Id}</b></p>
            <p>URL:</p><br/>
            {!v.windowURL}
        </div>
    </lightning:card>
</aura:component>


//JS
({
    init : function(component, event, helper) {
        var pageRef = component.get("v.pageReference");
        component.set("v.windowURL", window.location.href);
        if (pageRef && pageRef.state && pageRef.state.inContextOfRef) {
            var state = pageRef.state; // state holds any query params
            var base64Context = state.inContextOfRef;
            
            // For some reason, the string starts with "1.", if somebody knows why,
            // this solution could be better generalized.
            if (base64Context.startsWith("1\.")) {
                base64Context = base64Context.substring(2);
            }
            var addressableContext = JSON.parse(window.atob(base64Context));
            component.set("v.contextId", addressableContext.attributes.recordId);
        }
        
    }
})