windows 11 website contact form filling

The 1Password browser extension doesn't detect this form on its own, so it doesn't add its 1Password icon into the input fields as it does for login forms. However you can click the 1Password extension button in the list of extensions offered by the browser (It's recommended you pin it to be always visible). The 1Password extension window will open and automatically select your identity entry. At least it's what happened when I did it on that website form. Then I clicked the "autofill" button on the extension dialog, and name+email+phone was autofilled from my identity entry.

Thanks, I have the button pinned but when I click on the web page, all items are displayed rather than identities.
How do I change this behaviour as it is not obvious to me?

There is a drop down list above the list of items. If you have "All items" selected, all items will be displayed. Change the drop down list to "Suggestions" (or whatever it is called in English, I don't have English as browser language) to only displayed matching items.