How to underline part of text Javascript

Hi: I am trying to figure out a way to underline only part of a "string" (if i'm using that correctly) I have a paragraph that auto populates based on a custom calculation script. 

I used the follwoing in the script in the custom validation script section wich underlines the whole paragraph. I would like to only underline the first two words of the paragraph. 

var span1 = {};

span1.text = event.value;

span1.underline = true;

event.richValue = [span1];

Any suggestions or solutions. Thank you !