Plotly Custom Legend

Question

I have a plotly plot which looks like this:

The Code I am using is below:

fig = make_subplots(specs=[[{"secondary_y": True}]])
fig.add_trace(go.Scatter( x = pf['Timestamp'], y = pf['Price_A'], name ='<b>A</b>', 
                           mode = 'lines+markers', 
                           marker_color = 'rgba(255, 0, 0, 0.8)', 
                           line = dict(width = 3 ), yaxis = "y1"),  
                           secondary_y=False,)

fig.add_trace(go.Scatter( x = df['Timestamp'], y = df['Price_B'], name='<b>B</b>', 
                           mode = 'lines+markers', 
                           marker_color = 'rgba(0, 196, 128, 0.8)', 
                           line = dict(width = 3 ), yaxis = "y1") , 
                           secondary_y=False,)

for i in pf2['Timestamp']:
   fig.add_vline(x=i, line_width=3, line_dash="dash", line_color="purple", 
                 name='Event')

fig.update_layout( title="<b>Change over Time</b>", font=dict( family="Courier New, 
                    monospace", size=16, color="RebeccaPurple"),
                    legend=dict(
                           yanchor="top",
                           y=0.99,
                           xanchor="left",
                           x=0.01
                    ))

How can I add the entry in the legend for the event that is denoted by the vertical lines?