Hi step8601,
Yes, you are right. I will provide this request to our developers to implement this in one of the next updates(depends on priority features)
Until that, you can edit your button cell by double click on that cell(besides the button, not on the button), and then it will be shown all HTML. Here is a screenshot https://ibb.co/fvBgbGM
Also, you can use the “Insert HTML” button from Toolbar where you can insert link over GUI and then edit HTML over Code button <> where you can add this attribute.
Best regards.