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
to ensure that the selected text is visible.