How to Enable HSTS in WordPress Site

To enable HSTS using .htaccess use below code

Header set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"

A breakdown of the header:

Strict-Transport-Security
Forces HSTS on the domain

max-age
How long the header should be active in seconds

includeSubDomains
Includes subdomains

preload
Authorizes preload listing if eligible (covered below)

After that To submit your domain for preloading, visit HSTSpreload.org