test.cs

// load the template
textControl1.Load("template.tx", TXTextControl.StreamType.InternalUnicodeFormat);

// get the "outofstock" SubTextPart
SubTextPart subTextPart = textControl1.SubTextParts.GetItem("txmb_outofstock");

// create a MergeBlockInfo object
MergeBlockInfo mergeBlockInfo = new MergeBlockInfo(subTextPart);

// add the condition
mergeBlockInfo.BlockMergingCondition =
  new List<TXTextControl.DocumentServer.DataShaping.FilterInstruction>()
  {
       new TXTextControl.DocumentServer.DataShaping.FilterInstruction(
          "available",
          TXTextControl.DocumentServer.DataShaping.RelationalOperator.Equals,
          false
          )
  };

// store the condition in the SubTextPart
mergeBlockInfo.Apply();