In this page choose to add a subdomain that you configured in The DNS settings. Add the subdomain name and don't forget to choose to add the Letsencrypt certificate (this shows only when adding a subdomain to an already configured domain):

After pressing Add, Yunohost will update the server configuration to add the domain to the NGINX reverse proxy that routes all port 443 traffic to the applications and install the TLS Letsencrypt certificate:

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