How to add ads.txt to blogger.com?

Set up an ads.txt file

If your blog is monetized with third-party providers or you’ve manually integrated AdSense in your blog, you will need to manually set up the content of the ads.txt file.

1. Sign in to Blogger.
2. In the top left, click the Down arrow  .
3. Click the blog you want to set up an ads.txt file on.
4. On the left, click Settings.
5. Under "Settings," click Search preferences.
6. Under "Monetization," find "Custom ads.txt" and click Edit.
7. Click Yes.
8. Copy the settings from your third-party monetization provider and paste them in the text box.
9. Click Save settings.
10. Optional: To check the content of your ads.txt file, go to http:///ads.txt.

Note: If your blog is not monetized, or it’s only configured to use AdSense using the Blogger-AdSense integration, then you do not need to manually set up ads.txt. Blogger will do this for you. Read more about ads.txt for AdSense.

reference has been taken from https://support.google.com/blogger/answer/1269077?hl=en