mitmproxy/doc-src/features/setheaders.html

This feature lets you specify a set of headers to be added to requests or
responses, based on a filter pattern. You can specify these either on the
command-line, or through an interactive editor in mitmproxy.

<table class="table">
    <tbody>
        <tr>
            <th width="20%">command-line</th>
            <td>
                --setheader PATTERN
            </td>
        </tr>
        <tr>
            <th>mitmproxy shortcut</th> <td><b>o</b> then <b>H</b></td>
        </tr>
    </tbody>
</table>