Problem with opening mailto-links

I’m using K9 on a chromebook. Today I want to send a mail from a website. I klicked the button “send an email”. But the Browser (chrome) opens MS-Web-Outlook and not K9. On other sites it works. I investigated the links and found, that links with mailto… are opening correct in K9, but links with javascript mailto are redirected to Outlook. Searching the web I found something about protocoll handlers in chromebrowser. Here are registered Outlook and Gmail. When I remove Outlook the javascript-link gets openend in gmail. But I found no possibility to add k9. Any idea, how to add it?
Regards Ludger