It can be done with a C# script in which the OpenUrl method of the Application class is used for a Url parameter.

I also use Textmesh Pro for the button text, because it offers the possibility to add an underline style to the text and to set the color to a bluetone so that the text can be stylized like a web link.

What you can do after that is to disable the button image, so that only the text is visible as Weblink, but the functionality of the button click is still available.

See my social profiles here
G+: https://plus.google.com/+JayAnAm
Twitter: https://twitter.com/jayanamgames
Facebook: https://www.facebook.com/jayanamgames
Patreon: https://www.patreon.com/jayanam

Production Music courtesy of Epidemic Sound: http://www.epidemicsound.com