Create primaryTextFrame

The MasterSpread object has a primaryTextFrame property. I've enver used it but I would imagine it would be something like: 

myParentSpread.primaryTextFrame = myParentSpread.textFrames.add();

https://www.indesignjs.de/extendscriptAPI/indesign-latest/#MasterSpread.html

There's a property createPrimaryTextFrame, which is a creation property, that is, you have to set the property when you create a document. You can't change an existing frame to a primary frame. here's how to use it:

doc = app.documents.add ({
  documentPreferences: {
    createPrimaryTextFrame: true,
    // And whatever else
  }
);

Click on the symbol on the upper part of the left edge of the text frame once. It will change it to a primary text frame.