TransparencySettings

To add some feather or blending (all that what is accesable through the effects panel) you need to go deep into the transparencySettings.

var tf = (app.documents.add(
    {
      documentPreferences:{
        pageWidth:200,
        pageHeight:200,
        facingPages:false
        }
      }
    )).pages[0].textFrames.add(
    {
      geometricBounds:[20,20,180,180],
      }
    );
    
tf.contents = 'a';
tf.paragraphs[0].pointSize = 288;
tf.paragraphs[0].justification = Justification.CENTER_ALIGN;
tf.textFramePreferences.verticalJustification = VerticalJustification.CENTER_ALIGN;


tf.transparencySettings.blendingSettings.opacity = 10;
tf.transparencySettings.blendingSettings.blendMode = BlendMode.MULTIPLY;
tf.transparencySettings.featherSettings.mode = FeatherMode.STANDARD;
tf.transparencySettings.featherSettings.chokeAmount = 10;
tf.transparencySettings.featherSettings.cornerType = FeatherCornerType.DIFFUSION;