The sendkey function in LotusScript is not supported in the Domino Notes client.
The only way that I know to insert a horizontal rule is from the backend. And the only solution I know of is not real elegant without using the C APIs.
You must create a document or profile document with a rich text field. Add the rich text element that you want. (In this case a horizontal rule.) Get the document, item and append the data from source item using the appendRTItem method in LotusScript or Java.