Hello,
Thank you for contacting Rank Math today.
When you add a website correctly you should see an option to perform the verification with HTML like so:

If you are not seeing that, please make sure that your website is not already verified which in this case this step could be skipped.
Don’t hesitate to get in touch if you have any other questions.
Now, My website is verified in bing now what do? There is any other way via XML.
Hello,
If your website is already verified on Bing, you do not need to verify it again using the XML file.
Don’t hesitate to get in touch if you have any other questions.
Then What should I do with Bing Webmaster Tools textbox, Should I leave it blank?
And There is XML file, I have downloaded and opened that file there is some code Should paste that code in Bing Webmaster Tools Text box?
Check out XML file (screenshot)
https://i.ibb.co/7zHkbfR/Capture1.png
Hello,
You can leave that field blank since your website is already verified on Bing there’s no need to have any data in that field.
Don’t hesitate to get in touch if you have any other questions.
Thanks to Miguel and Anas (Rank Math Team)
Hello,
We are super happy that we could address your concern. 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.