Have you tried setting the value like this?
Base.Opportunity.SetValueExt<CROpportunityExt.usrExtArea>(row, contactExt.UsrExtArea);

Have you tried using the following code?

Base.Actions.PressSave(); instead of Base.Save.Press();

I don’t see any issues with your code. There are multiple ways you could be doing it, but it looks fine to me. I wonder if you’re not fighting the workflow engine, since the Open action is a workflow action. Have you tried executing the baseMethod before your code?

public IEnumerable open(PXAdapter adapter, openDelegate baseMethod)
{
    var row = Base.Opportunity.Current;

    var result = baseMethod(adapter);

    // Your code

    return result;
}

thank you all I solve it. 

Hi @vannakheng66 , Can you share how you solved it? Thank you!

In Opportunity we have projection so we need to extend that project class more.