There is an easier workaround to send the user directly to the edit form, use a redirecting display form with some simple JavaScript: https://www.dmcinfo.com/latest-thinking/blog/id/8674/always-open-sharepoint-2013-tasks-in-edit-mode
I spent ages looking for a way to get the taskID so I could send the approver directly to the form but there is no way to do it, this is the best workaround i've seen.
Cheers, Anros