Set default value in item master for tax

Hello, i can’t set default value for tax in item master customize form. I think problem is that item tax field is table type. I wan’t to set value by default when creating anew item. Like on picture

if(frm.doc.__islocal) {
		var row = frm.add_child("taxes");
		row.tax_type = "PDV 21% - GC";
		row.tax_rate = 21;

You can do something like this and make it run on load, so that if its a new document a row will be added in the child table.