Setting keyword entry in opensearch.xml

When I check the firefox settings for searchengines I can see a column for keyword

In the opensearch.xml I can define Tags which does not create an entry here. When I add my own opensearch, I want to also set the "Keyword". How do you do that?

My opensearch.xml looks like this so far and is working (I can add the search in firefox over the green plus icon in the search bar):

<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/"
                       xmlns:moz="http://www.mozilla.org/2006/browser/search/">
    <ShortName>MyTitle</ShortName>
    <Description>My Description
    </Description>
    <InputEncoding>UTF-8</InputEncoding>
    <Tags>mytitle</Tags>
    <Image width="16" height="16" type="image/x-icon">https://myexample.com/favicon.ico</Image>
    <Url type="text/html" template="https://myexample.com">
        <Param name="query" value="{searchTerms}"/>
    </Url>
    <moz:SearchForm>https://myexample.com</moz:SearchForm>
</OpenSearchDescription>

I couldn't find any references in the docs.