How to add alt-text to SiteOrigin buttons?

We are using the SiteOrigin Button widget to add buttons to our website and cannot figure out how to add alt-text for these buttons. For example, the button on the page I’m linking to says “Continue to Learning with Digital Media.” We want to add alt-text for people using screen readers that says the same thing.

URL: http://digitalmediaeducation.org/2-1-week-2-introduction/