You will want to create a relationship where each Milestone can have many activities.
It will by default create a button for you to "Add Activity"
You will want to modify that formula-url field to include and automatically complete the Project along with the respective milestone.
In this example the Record ID is getting written to field id 15, and the related project it getting added to field 14.
(This button will be on the "Milestones" table).
URLRoot() & "db/" & [_DBID_ACTIVITIES___ACTION_ITEMS] & "?a=API_GenAddRecordForm
&_fid_15="&URLEncode([Record ID#])&"
&_fid_14="&URLEncode([Related Project])&"
&z=" & Rurl()
You will most likely need to add the portion:
&_fid_14="&URLEncode([Related Project])&"
So you can pre-populate the form with the related Project AND Milestone
