because Opportunities use the MutableEntityFieldDescriptor there are no legacy functions to retrieve the Default value, which is not the case for Contact, Companies, that use the former 'FieldDescriptor'
As such getting the default value now has some more steps.
Here is the code for Opportunity field default value
Act.Framework.MutableEntities.MutableEntityFieldDescriptor uField2;
Act.Framework.MutableEntities.DefaultFieldValueAttribute uFieldDeflt;
AttributeCollection uAttcol;
string TB1 = "fieldName"
uField2 = Classes.Act_Findfunctions.FindOppFieldDesriptor(TB1, this.actApplication);
uAttcol = uField2.Attributes;
uFieldDeflt = (Act.Framework.MutableEntities.DefaultFieldValueAttribute)uAttcol[2];
string uStr = Convert.ToString(uField2.GetValue(currentRec));
if ((uStr == null) | (uStr == "")) { uStr = Convert.ToString(uFieldDeflt.DefaultValue); }
this.tbTaxPct.Text = uStr;
DBS