The problem looks to be that you are scheduling the very first task from "today"... If you link them nose-to-tail the entire project will schedule from "today" and the milestone _must_ move to accommodate that.
If you set the start date of the first task sufficiently far in the past that there is enough time to fit all the first tasks in before the milestone then all will be well. This might mean you have to set the Project Start Date in the past too.
Once you have done all that:
1. Fix the start of the milestone by making it a "Must start on..." or "Start no later than..."
2. Make the milestone a dependant of the last task in the first section (i.e. make the final previous task a predecessor of the milestone).
3. Make all the prior tasks "Finish as late as possible"
Hope that helps.