When I was developing in Lotus Notes/Domino, we can use such events like QuerySave that allow us to run some code before saving and closing the form.
I thought it should be easy in Nintex Form to develop such a functionality.
Good news: No need to use two buttons One is enough.
If you need to update only one field, there is a standard feature.
In the Control Settings of the button, under Advanced, for Connected to,
you can select one field and fill the value returned: Completed
End of update (2015/10/27)
But if you need to update more than one fields, here is the complete solution for O365. I believe it should be the same for on-Premise.
Let's say we want to update to 'Completed' a drop down box containing Statuses.
NWF$('#' + fldStatus).val("Completed").trigger('change');
Expand Advanced, and for Client Click, put MyFunction()
"Make mistake but learn from them"