Display method + AX 2012

Create a method display in the DataSource TestTable of your form and use this code for example:

Display string30 VendAccount(TestTable v = testTable)
{

    string30 VendAccountExt;
    ;
        if(TestTable.AccountNum!="")
        {

            VendAccountExt = v.AccountNum + "Test";
            return VendAccountExt;
        }
        return VendAccountExt;
}

Then in the proprities of the filed set this method as a DataMethod.