Need some help with SSL for sub-domain

My domain fitzmaurice.works is with Google Domains. I have created a CNAME sub-domain news.fitzmaurice.works to redirect to fitzmauriceworks.moosend.com, this is an email marketing service similar to Mailchimp.

I've verified the perosnalised domain new.fitzmaurice.works on their platform but when I browse to it I get a warning the site is not secure.

I have been in touch with both Google and Moosend. Moosend have offered to help but say they will need to implement the SSL for my domain in Moosend but need a SSL certificate. They say I need to create a PFX file in WINDOWS format of that certificate and share with them.

Google Domain Support said this is outside their remit and referred me to my webhost, i.e. Squarepsace. Squarespace support say they only handle SSL pointing towards fitzmaurice.works so I am not really sure where I go from here.

Hoping someone recognises this scenario and can help. See pic of issue attached.

Robert