Scripting a Photoshop Action: Convert Image to Indexed Color (Perceptual) and Save Color Table

I am not aware of DOM or AM code to save an .act file via scripting. Perhaps someone else can comment.

Where the DOM is lacking, there is AM (Action Manager) code:

var idCnvM = charIDToTypeID( "CnvM" );
    var desc5609 = new ActionDescriptor();
    var idT = charIDToTypeID( "T   " );
        var desc5610 = new ActionDescriptor();
        var idPlt = charIDToTypeID( "Plt " );
        var idClrP = charIDToTypeID( "ClrP" );
        var idAdpt = charIDToTypeID( "Adpt" );
        desc5610.putEnumerated( idPlt, idClrP, idAdpt );
        var idClrs = charIDToTypeID( "Clrs" );
        desc5610.putInteger( idClrs, 128 );
        var idFrcC = charIDToTypeID( "FrcC" );
        var idFrcC = charIDToTypeID( "FrcC" );
        var idBanW = charIDToTypeID( "BanW" );
        desc5610.putEnumerated( idFrcC, idFrcC, idBanW );
        var idTrns = charIDToTypeID( "Trns" );
        desc5610.putBoolean( idTrns, true );
        var idDthr = charIDToTypeID( "Dthr" );
        var idDthr = charIDToTypeID( "Dthr" );
        var idDfsn = charIDToTypeID( "Dfsn" );
        desc5610.putEnumerated( idDthr, idDthr, idDfsn );
        var idDthA = charIDToTypeID( "DthA" );
        desc5610.putInteger( idDthA, 75 );
    var idIndC = charIDToTypeID( "IndC" );
    desc5609.putObject( idT, idIndC, desc5610 );
executeAction( idCnvM, desc5609, DialogModes.NO );

Or put through the CleanSL script:

var c2t = function (s) {
	return app.charIDToTypeID(s);
};
var s2t = function (s) {
	return app.stringIDToTypeID(s);
};
var descriptor = new ActionDescriptor();
var descriptor2 = new ActionDescriptor();
descriptor2.putEnumerated( s2t( "palette" ), c2t( "ClrP" ), s2t( "adaptive" ));
descriptor2.putInteger( s2t( "colors" ), 128 );
descriptor2.putEnumerated( s2t( "forcedColors" ), s2t( "forcedColors" ), s2t( "blackAndWhite" ));
descriptor2.putBoolean( c2t( "Trns" ), true );
descriptor2.putEnumerated( s2t( "dither" ), s2t( "dither" ), s2t( "diffusion" ));
descriptor2.putInteger( s2t( "ditherAmount" ), 75 );
descriptor.putObject( s2t( "to" ), s2t( "indexedColorMode" ), descriptor2 );
executeAction( s2t( "convertMode" ), descriptor, DialogModes.NO );

In this case, the code was recorded via the ScriptingListener plug-in:

https://helpx.adobe.com/photoshop/kb/downloadable-plugins-and-content.html