You can show a tooltip programmatically using the ImageryLayer_MarkerSelected event:
imageryLayer.MarkerSettings.TooltipSettings.ShowTooltip = true;
imageryLayer.MarkerSettings.TooltipSettings.TooltipTemplate = new DataTemplate(() =>
{
StackLayout tipLayout = new StackLayout();
tipLayout.HorizontalOptions = LayoutOptions.Center;
tipLayout.VerticalOptions = LayoutOptions.Center;
tipLayout.Padding = 0;
tipLayout.Margin = 0;
Label mylabel = new Label();
mylabel.Text = e.MapMarker.Label;
tipLayout.Children.Add(mylabel);
return tipLayout;
});