Highlight certain text in textbox

Hi,
I have 2 text boxes.
For example:
textbox1 = "Hello bob, How are you"
textbox2 = "Hello cad, how are you"


I have looped through each text box and know that the area I want to highlight on textBox 2 is between the charaters 7 and 9. How do I highlight only these characters as Bold and with a yellow background?
Thanks

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