Pegasus

Hi Guys, I really need some help connecting my ionos webmail calendar I need it to sync it with my Squarespace scheduler so that when I send a link to prospects they can book in only available time slots. Squarespace advisers have sent me a link to the same FAQ page and advised me that it will only sync with the third party providers listed which ionos is not one of. I know this can be done because it took me ages to figure out how to do it last year, then I had to cancel my subscription to scheduler. I can't for the life of me remember what I did to get them synced. Can anyone help?