How to get task ID related with list item

Hello,

I have a list workflow and I do not want the approver user go to standard form task to approve item, I want create a custom form and let the user select Approve o Reject and programmatically (Using REST + JSON) update the workflow Task and then workflow continue.

There is some way to get the task ID related with the list item. 

In forums I found the way to get the item id related whit some task, but I need exactly the oppose 

Thank you.

SharePoint