HSTS & Subdomain Issue

Hi, I recently set up my domain to use HSTS, i added the header into my .htaccess file which includes subdomains and preloading, then i successfully submitted the domain to the HSTS preloading list. All is working perfectly on the root domain and i have no issues. www/non-www work fine and redirect to my prefered url all through https. Everything is setup correctly. My issue is now that i have decided to setup a new subdomain for a shop front. (i tried to use a subfolder instead but there is overlapping script errors and so i think using a subdomain would be the solution) This subdomain is apparently protected, but through a self signed certificate. I cannot access said subdomain because the HSTS is not allowing me to do so. How would i go about getting access to this one subdomain or setting up HSTS for it? Do i need to purchase another seperate SSL certificate? Thank you.