You don’t have to make a post to claim a character you only need to have an activity page for the character you wish to claim. This can be done by switching to that character on the forums, or if you don’t trust that one simply like a post.
Blizzard Forum Activity
The second way is through using Blizzard Forum activity. This way requires the character to exist on Warcraft Logs before you can claim. If you can find the character page through the search bar, you’ll be able to claim it to your WCL account.
To claim a character on the site, you need to create a Blizzard forum activity page for your character. All you need to do is go to the Blizzard forums and select your character. This will create the activity page and you’ll be able to use the form at the bottom of this page.