How to set Milestone to confidential

How to set Milestone to confidential

Hello, I created milestone under public group.
People outisde of organization could not see the issue from private project however they can see the milestone.

I understand that if set milestone under private project, people outside of organization could not see the milestone however if the milestone is set under project, it could not link issue of another project in one group.

Is there a way that we can set milestone under group (so we can link issues of any projects under same group) but it also can set to confidential (so people outside of organization could not see the milestone)

Notes: we couldn’t set group under private since we have project that is both public & private