In the website editor, go to the Settings menu (top right corner) → the Scripts section. Add the code line in the Before </head> field. Normally, the confirmation code is as follows:
<meta name="yandex-verification" content="your ID in Yandex system" />
or
<meta name="google-site-verification" content="your ID in Google system"/>
Apply the changes and publish the website to put them into effect.