Use the ugly link instead of the pretty link. For example, if your post id is 123, you can link to http://www.yoursite.com/?p=123. This will automatically get rewritten to your current pretty permalink.
If you do it relative from the root, it also keeps the link intact if you change your domain name. (ex. href=”https://wordpress.stackexchange.com/?p=123″).
To find the post ID, hover over its “edit” link in the WordPress admin. Look at the target URL, and you’ll see a “post=1234”. The number is your post ID.