Where to get the newsletter subscription link?

Check the left-side navigation panel Settings and then select Sites. When you click it, you'll reach a view showing one or more sites. Usually, the site name is in the format newsletter.customer-own-domain.com or similar. You can type the address into your browser, where the subscription option should appear. You can also click a site, and in the next view, there's a "go to page" section.​