Hi,
You have written your code on Pre-operation plugin?
Below code should work
if(target.Attributes.Contains("FIELD_1_Schema_Name") && target.Attributes.Contains("FIELD_2_Schema_Name"))
{
target["FIELD_3_Schema_Name"]=target["FIELD_1_Schema_Name"]+target["FIELD_2_Schema_Name"];
}
Please mark my answer verified if i were helpful