It is possible to write into this property only if the PmForm window has not been opened yet.
This property is also functional in Web panels.
//Setting the propert when the object is created
var oBtn = oForm.CreateItem("button", "Id1", "Title", "ValueHorzAlign:right;");
var sValue = oBtn.ValueHorzAlign; //Reading the value
oBtn.ValueHorzAlign = "right"; //Writing the value