Search textbox using checkbox in Javascript

I am trying to do a text-search with a checked checkbox. For example, if the person selects the check box the input will diplay. Say I enter "the" it will search for all "the" in the paragraph and will highlight all the "the". I already go that part down what I am not understanding is how to make the checkbox connect with the text-search form. So when the user selects the checkbox "the" will display(or whatever word/letter they search for. Thank you

I was thinking of using the if statement...