Hi,
There are multiple ways of doing this. You can either install a call to action plugin with this functionality, or write your own script and markup which you add in a Hook Element,
As for plugin recommendations, we don’t specifically have a plugin to recommend but as long as it is well-coded and works on default WordPress themes, then it should work with GeneratePress.
As for custom markup and script, we don’t provide one as this is a site customization that is outside of our scope of support.