It would be nice to be able to have a Share button on my blog using ShareOpenly service.

Here how it should work:

Any idea on how to implement it?

It looks to me like you’d take the exact format of the link on that page you linked to:{URL}&text={TEXT}

And you’d put it in a template after each post, but instead you’d put {{ .Permalink }} where it says {URL} above and omit the &text={TEXT} part, I assume.

There’s very little “documentation” for that service, but it’s just a link format.