presenting evidence of research to grad school committees
While a link to a website is probably fine, it might not be followed up, so make your short description clear. A link to a page on a university site or other obviously "safe" domain is preferred, of course. 

And, if you have a CV to submit, you can mention it there as "unpublished undergraduate research". 

But the "best" way is to have one of your letter writers mention it in their LoR. This is especially true if the project supervisor is one of the writers. But another professor who knows of the work can also make a brief mention.