Wildcard Let's Encrypt

Hi

I have an existing Laravel build using Forge and Digital Ocean.

I want to use Spark to allow me to take Stripe payments easily.

My understanding is that you cannot incorporate Spark in to an existing Laravel build so I was thinking to create another site on the same DO server using Forge and have it as a subdomain. mysite.com, sales.mysite.com

Now that Let's Encrypt can handle Wildcards where would I create the Let's Encrypt certificate? On the main site or the sub domain site?

Hope that makes sense? :-)

Thanks

Andy