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HSTS
HSTS is short for HTTP Strict-Transport-Security. It is a defined way for a server to tell a client that the client should prefer to use HTTPS with that site for a specified period of time into the future.
Here is how you use HSTS with libcurl.

In-memory cache

libcurl primarily features an in-memory cache for HSTS hosts, so that subsequent HTTP-only requests to a host name present in the cache will get internally "redirected" to the HTTPS version. Assuming you have this feature enabled.

Enable HSTS for a handle

HSTS is enabled by setting the correct bitmask using the CURLOPT_HSTS_CTRL option with curl_easy_setopt(). The bitmask has two separate flags that can be used, but CURLHSTS_ENABLE is the primary one. If that is set, then this easy handle how has HSTS support enabled.
The second flag available for this option is CURLHSTS_READONLYFILE, which if set, tells libcurl that the file name you specify for it to use as a HSTS cache is only to be read from, and not write anything back to.

Set a HSTS cache file

If you want to persist the HSTS cache on disk, then set a file name with the CURLOPT_HSTS option. libcurl will read from this file at start of a transfer and write to it (unless it was set read-only) when the easy handle is closed.
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In-memory cache
Enable HSTS for a handle
Set a HSTS cache file