Okay ... just so people know what 'side' I'm coming from, I'm a client, not a designer.

Have clients where appropriate (web work) link to designers websites/portfolio sites.

So the winning designer of a webpage/template/web logo etc would have a link to their site at the bottom of the page.

This would be a simple text link as you see on many sites.

I don't think this could be compulsory, but perhaps a default selection when clients sign up - so clients have to opt-out of this choice.

Not every client will want this - personally I'm happy to offer this. Perhaps as a 'sweetener', the designer (should they want to promote their website), could have the option of offering a discount should their link be published.