could not insert text inside textbox in ms word programably

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C#

hellow
i want to write a text inside a textbox that i have created in ms word(programably by c#)

xShape  = oDataDoc.Shapes.AddTextbox(Microsoft.Office.Core.MsoTextOrientation.msoTextOrientationHorizontal, 1, 1, 75, 50, ref rng);
object ss2=(object)xShape;
xShape.Select(ref ss2);

using the above code i had to close my c# program in order to make the textbox active in word document so i can write my text inside it
OR
i have to press in the mouse anywhere on the document in order to make it active
BUT MAY BE ANOTHER SOLUTION
if i could insert a bookmark(B1) inside this textbox using

book1 = oDataDoc.Bookmarks.Add("B1", ref ss2);//<--- did not worked i get type mismatch

the 2nd paramter is my problem, how could i get range of the textbox ?