According to the API Documentation, you can create a new milestone using the command newFixFor.
I'm using the command, and it returns the created milestone successfully. However, when I go into fogbugz and view the milestone under projects, it is unassignable. I am passing in the fAssignable=true into the url so I don't understand why the milestones end up being unassignable.
Here is the query string
'cmd=newFixFor&fAssignable=true&ixProject=' + projectId + '&sFixFor=' + '\"' + milestoneName + '\"'
What's the problem?
It's fAssignable=1, not fAssignable=true.