how to add approve or reject on a nintex form where a user is able to have all actions available in the form rather than having to go to a separate link to approve
that' not possible.
task approve/reject controls are only available on the task form.
you should understand the difference between the task form and list item form.
you can simulate the same using your own form. If you want to use the same form for approval, just add different buttons on the form. Connect each button to field with a value so that you can manage different stages of the form. In this case you will be managing the workflow. The concept of task or task list will not be there as Marian suggested.
I don't think this is a good approach.
- it will make workflow significantly more complex since every single task action will need parallel handler to process changes made on the list (plus complexity to identify proper task if flexi task action is used)
- it will be really complicated to manage user uses button on list form in proper time range, ie. while task active. if it's used too early you would need extra check/handling, if it's used too late you might missed the decision or it may lead to unexpected scenatios
- it might be problem to manage permissions who can approve the task
- if there are several assignees or group of assignees is dynamic, you will not be able to evaluate different outcomes
- etc. - I believe plenty of other obstacle will pop up
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