In order for you to be able to request a wildcard LetsEncrypt certificate you will need to use any of the supported DNS providers. At the time of writing, this is Cloudflare, Vultr, Linode, Hetzner & DigitalOcean.

You can link any of these DNS providers inside your profile under the Integrations tab. Of course, you can also manually enter these details when requesting a wildcard SSL certificate.

To request a wildcard Let's Encrypt certificate you will have to request the certificate wild the wildcard character: *

An example of how you can achieve this: domain.com,*.domain.com