Requires the use of the .org companion plugin. IE not designed for section use.
To set up the Modal Join block, you will first need to ensure you have Join Links set up for each of the Countries, under the Network Options:

Once you have added your Join Links, you can then insert the Modal Call to Action block within any page.
You can edit the Heading, Content & Button text within the Modal by clicking the button within the editor:

When previewing on the front-end, when a user selects an option from the dropdown, they will automatically be redirected to the join page for that chosen country. They will not need to click the Join button for this.