I have an HTTPS server in Go, so that users can enter https://example.com in their browser. I'd like them to also be able to type in "example.com" to reach my HTTPS server. From this answer I see that the standard solution is to run an HTTP server that redirects to https://example.com.
I'm wondering if this is as secure as if everything were in HTTPS. In particular, if a user sends a POST request to "example.com", will the content be encrypted while it passes through the network?
A POST request to an HTTP server is not secure. This includes the case where the server responds to the POST with a redirect to an HTTPS server.
Browsers automatically follow redirects. The request to the HTTPS server is secure.