How to enable Collection signing with manual signatures

I want to upload signatures to my collection and it seems I have to enable collection signing before that.

The instructions at Enabling collection signing - Galaxy NG instruct to “Create a signing service”, etc…

I however don’t want any automatic signing. I only want to sign manually with key I have on my smart card whenever I make a release. I already have a .asc detached signature ready to go. How can I enable collection signing and upload the signatures?