man curl:

-H/–header
(HTTP) Extra header to use when getting a web page. You may specify
any number of extra headers. Note that if you should add a custom
header that has the same name as one of the internal ones curl would
use, your externally set header will be used instead of the internal
one. This allows you to make even trickier stuff than curl would
normally do. You should not replace internally set headers without
knowing perfectly well what you’re doing. Remove an internal header
by giving a replacement without content on the right side of the
colon, as in: -H “Host:”.

curl will make sure that each header you add/replace get sent with
the proper end of line marker, you should thus not add that as a
part of the header content: do not add newlines or carriage returns
they will only mess things up for you.

See also the -A/–user-agent and -e/–referer options.

This option can be used multiple times to add/replace/remove multi-
ple headers.

Example:

curl –header “X-MyHeader: 123” http://www.google.com

You can see the request that curl sent by adding the -v option.