Go to the Muck Rack homepage here: https://muckrack.com. Then, click the "Login" button at the top right corner of the page and log into your account.
Once logged in, click on the "Chat" icon at the bottom right corner of their page to connect with chat support.
On the chat window, click the "Send us a message" to start a new conversation.
Type in the chat box that you want your account deleted, then click the Send button.
The chat support will notify you once they're done deleting your account.