The only highlight you can apply to a TextBox is Selection - just as if you had double clicked a word - there is no specific Highlight. For that to work properly, you need a RichTextBox.
To select specific text in a TextBox is easy:
myTextBox.SelectionStart = indexOfFirstCharacterToSelect;
myTextBox.SelectionLength = numberOfCharactersToSelect;
You may need to add
myTextBox.ScrollToCaret();
to ensure that the selected text is visible.
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