To understand how this works, imagine that the ItemsSource is a list of persons with various informations divided into different properties, like Names, Surname, Age, etc., all contained within a Persons object. The DisplayMemberPath property could be set to target the Name property held within the Persons object so that the ComboBox displays the names of the persons listed in said Persons object.

Then, the FilterTextPath property could be set to target the Age property of the same Persons object so that the ComboBox displays only the names of the persons corresponding to a specific Age value when that value is typed in the ComboBox.

Note that if no value is specified for the FilterTextPath property, the items displayed in the ComboBox will only be those put forth via the DisplayMemberPath property, without additional filtering done by the FilterTextPath property. 

In the following example, the ItemsSource uses a binding on the UserObjects property of the ContentPage, which is a collection of objects representing various hockey players.

Each object, meaning each players in the collection, has various attributes stored in properties, like Name, Team, Goals, etc.; as their names suggest, those properties contain various informations about each player in the collection.

Here, the DisplayMemberPath is used to retrieve the data found in the Name property so that the ComboBox can display the names of the players in the collection.

Then, the FilterTextPath is used to filter those players using the data contained within the Goals property, which, in this case, corresponds to the number of goals each player has scored. Therefore, a user could type in "5" in the ComboBox & the Popup would display only the name of the players who have scored 5 goals because the ItemsSource's data is being filtered by both the Names & Goals properties at the same time.

<xctk:ComboBox WidthRequest="125"
               ItemsSource="{Binding Userobjects, Source={x:Reference _contentPage}}"
               DisplayMemberPath="Name"
               FilterTextPath="Goals"/>

Target Platforms: Windows 11, Windows 10, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference

ComboBox Class
ComboBox Members