Adding a Dropdown to Timeline

First, create a new dropdown category from Administration > Data Management > Dropdowns

Second, add your values to this new dropdown category

Third, edit the Timeline Activity type in Administration > Timeline

Fourth, use the “+Add” in the editor to add a new field; name the field, select field type of dropdown, then enter the name of the dropdown category you just created.

Fifth, add the new field to the layout and rearrange the placement to be where you want. Then Save

I did it!!  haha

Thank you:) @john_apple