for your usecase it seems like you have to generate a custom control template.

For reference here someone already tried to create a custom combobox. custom combobox in wpf Application

If you'd like to use the original design, you can find the source here: https://learn.microsoft.com/en-us/dotnet/desktop/wpf/controls/combobox-styles-and-templates?view=netframeworkdesktop-4.8#combobox-controltemplate-example

Hope I was able to help you with those sources for now