Hello,
Thank you for contacting Rank Math, and sorry for any inconvenience caused.
Assuming you are referring to the sitename displayed in the preview. Please head over to WordPress settings > General > Site title and add your sitename there.
If you are referring to the site name displayed in posts, you need to use %sitename% variable.
You can add this variable after the %sep% variable.

Hope this helps.
Don’t hesitate to get in touch if you require any further assistance.
Thanks buddy, This works.
Hello,
We are super happy that this addresses your issue. If you have any other questions in the future, know that we are here to help you.
If you don’t mind me asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post about your overall experience with Rank Math? We appreciate your time and patience.
If you do have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.
Thank you.