Calling custom WEB api from JavaScript

Hi 

We have written new Web API service and it has POST methods.  We deployed it in one server.  CRM domain is enabled from this server.

We are trying to access API this API from HTML webresource in CRM using Javascript  but not able to call and getting below error

XXXXX_Jquery-3.3.1:9600
Mixed Content:
The page at 'abcd.dynamics.com/.../ABC.html' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'testdomain/.../TestApi'.
This request has been blocked; the content must be served over HTTPS.

We have tried to provide the self sign certificate getting error - 

• failed to load resource: net::err_cert_authority_invalid

Could please let me know how to resolve this ? 

Thanks