How to extract a string from a TextRange

Dear Experts,

How can I extract the string "=?utf-8?B?LDvGhsLCBGcmybYk?=" between the coding markers?

Currently, I am able with the help of code like

startText = activeDoc.Find(startText.beg, findParams);

to find the exact start and exact end of the string and I have created the TextRange Base64Text.

With the instruction activeDoc.TextSelection = Base64Text; I can highlight the text in my document.

I have tried further with

Base64Item = activeDoc.GetTextForRange(Base64Text,Constants.FTI_String);

Base64String = Base64Item.toString();

But still I am not able to see the string in the object browser of the Toolkit.

Note that I don't use the method GetText because the string is only a part of the paragraph.

Thank you for advices.