Here is a simple Xamarin Forms sample app that demonstrates how you can attach a behavior to an Entry element and move the focus to another element when the user clicks enter on the keyboard.
Give the element you wish to focus on a name:
<Entry x:Name="Entry2" Placeholder="Field 2"/>
On the element you wish to navigate from, add the behavior and add a reference to the named element.
<Entry Placeholder="Field 1">
<Entry.Behaviors>
<behaviors:SetFocusOnEntryCompletedBehavior TargetElement="{x:Reference Entry2}" />
</Entry.Behaviors>
</Entry>
When the user presses enter whilst on "Field 1", then focus will moved to "Field 2"