Placing an onmouseover expression in HREF field

I would like my Contribute clients to be able to enter tooltip text via the HREF tag. I want the A tag to look like: <a href="#" onmouseover="Tip('Some text')">Homepage </a>

I've tried many combinations but the HREF field always reads the onmouseover="Tip('Some text')" text as part of the link rather than a separate expression.

Anyone know if this is possible?