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 your Cloudflare account in your profile under "Integrations", the Linode or DigitalOcean is used from your added server provider API key.

Of course, you can also manually enter these details when requesting an wildcard SSL certificate.

To request a wildcard LetsEncrypt certificate you will have to request the certificate with the wildcard character (*).

A example on how you can achieve this: 

domain.com,*.domain.com

Make sure to also include the original domain as that is the root domain.