Here's a demo extension that will do what you want for Gmail.
Create a folder named "Send to" on your desktop.
In the folder, put these 2 files (that you create in a text editor):
manifest.json:
{ "manifest_version": 3, "name": "Send to with Gmail", "version": "1.0", "permissions": [ "contextMenus" ], "background": { "service_worker": "background.js" } }background.js:
chrome.runtime.onInstalled.addListener(() => { chrome.contextMenus.create({ id: "sampleContextMenu", title: "Send to with Gmail", contexts: ["selection"] }); }); chrome.contextMenus.onClicked.addListener((info, tab) => { if (info.menuItemId === "sampleContextMenu") { const selectedText = info.selectionText; if (selectedText.includes("@")) { const mailto_url = "mailto:" + encodeURIComponent(selectedText); const url = "https://mail.google.com/mail/?extsrc=mailto&url=" + encodeURIComponent(mailto_url); chrome.tabs.create({url: url}); } else { const error_message = "data:text/plain;charset=utf-8," + encodeURIComponent('"' + selectedText + '" does not seem to be an email address.'); chrome.tabs.create({url: error_message}); } } });Then, in Opera, goto the URL opera://extensions, turn on developer mode, click "load unpacked" and point it to the "Send to" folder on your desktop.
Then, when you hightlight a plain text email address on a page and right-click the selection, there will be a "Send to with Gmail" option in the context menu that you can click.
You can of course edit the name of the extension and the name of the context menu item etc., but that should get your started.