In the form view: 

1. Add an existing field: Next activity deadline

2. Add a new field: date

3. Copy the Technical Name of the field “Next activity deadline” (activity_date_deadline)

4. Go into the date field > More > in “related field”: put the Technical Name.

5. Make the field “date” invisible

In automations:

Create an automated action in the model: Lead/Opportunity

1. Action to do: Update the record

2. Trigger: on Update

3. Trigger fields: empty

4. Before update domain: Stage > Stage name: contains “New” (for example)

5. Apply on: 

1. Stage > Stage name: contains “New”

2. “date”: is defined

3. Next activity type: contains “Meeting”

6. Data to write: 

1. Field: Stage (Lead/Opportunity)

2. Evaluation type: Value

3. Record: “Meeting” 

4. Value: 2