apply font on certain portion of text in textbox

I'am trying to make a portion of text in a textbox bold and the rest of the text stays normal.The textbox is in excel of course I've used this code to create it.

Microsoft.Office.Interop.Excel.Shape t1 = ws.Shapes.AddTextbox(Microsoft.Office.Core.MsoTextOrientation.msoTextOrientationHorizontal, 20, 15, 200, 77);

I tried doing that by adding html tags but it seams useless because they just behave as normal text. I appreciate some help thanks;