Hi @mradziak it is not possible to add the Approval URL reference to a Send Notification action since there is no assigned task to inherit this information from. You have notification/reminder options within task actions that offer this already. If you need to send the task URL out in a Send Notification action you will have to build this URL manually. You can capture the Task ID in a variable (as output from the task action) and append that ID to the URL for your task list. For example:

https://[SiteURL]/_layouts/15/NintexWorkflow/ApproveReject.aspx?List=[ListID]&ID={MyTaskIDVariable}

Please note that this Task ID variable is only available after the task action is completed, so this Send Notification action would actually lead to the completed task, rather than a pending one.