From the Milestone summary, click on the 'Record First Emergence' link (see red box 1 in the figure below).
This will open the next popup window to add the date of Emergence as seen below:
Choose the date at which the first Emergence was seen across the field.
Please note, the calendar will open on the current date.
The same may be done for the 50% emergence. Once added, the Yield dig date will be calculated and displayed.