Hi there @Henrik 8k ,
Welcome to the Community.
I have investigated further and I believe this is because our Hosted Pages do not support SSL certification from within your account at this time. This is something we have open feedback with our engineering team to offer but as of right now we do not have a timeframe when this will be evaluated and worked on.
While you can't enable an SSL certificate with your account, you can use an HTTP proxy to handle SSL certification (thus resolving the warning from being presented on the hosted pages). This can be done via a CDN like Cloudflare, Fastly, Cloudfront, etc...
For example, Cloudflare is a free provider of such a service. Once you've signed up for a Cloudflare account (or similar provider) and connected your site, you can manage DNS records within Cloudflare. Add a CNAME record that points to your desired hosted page and make sure the option for HTTP proxy is included (orange cloud icon vs. grey cloud icon).
You may need to toggle the cloud icon on/off after adding the record. Navigate back to the hosted page and observe that the security warning is no longer present.
The steps are similar to our guide on setting up SSL for dedicated click tracking.
Thanks!
Alex