I retired from office work in 1988 so this might be a bit too formal for today and I'm not familiar with 'content marketer'.
Hello,
I am abc, a keen content marketer working in the lifestyle, health and well-being fields and am really impressed with the efforts you have put into (websitename), reading it almost every day. I should very much like you to promote my brand on your website.
If this is possible, and subject to your approval of it, please let me know how much you would charge for publishing an article of around xxx words.
Looking forward to hearing from you.
Regards,
abc