Also, when making links, do so like this: "text that people see." :/posts/3513213 (but no space between the quote and colon) It appears as text that people see.
If you have the domain name in the link, it will change which domain people are using, and not everyone is on danbooru.donmai.us
When you just want to show the post number, just type post #3513213 and it makes a link automatically.