Underlining particular part of string

Dear Gurus.

Can any one let me know how to underline particular part of string?. My requirement is i have description text box in my wdp screen. when user enter's description, based on some
condition,  i need to append his sap login id and username to description and submit the request.

So how to underline SAP login id and username concatinated to description?.

PS . the below snapshot for reference.

Thanks

Santhosh