I have a comboBox with a ComboBox.ItemTemplate
<ComboBox>
<ComboxBox.ItemTemplate>
<DataTemplate DataType="{x:Type ViewModel}">
<TextBlock Text="1" />
</DataTemplate>
</ComboxBox.ItemTemplate>
</ComboBox>
The Item renders properly, but when I try to select the ComboxItem, the selection is only available on “1” rest of the Area in the ComboBoxItem is not clickable.
How do I set up the code so the selection can happen on the whole item and not just the Textblock.

Your ComboBox/ComboBoxItem seems to be templated, by default the highlight brush is a deeper blue. If you have access to the templates make sure the content of the ComboBoxItem stretches horizontally.
If the template is bound “correctly” setting the content-alignments may be enough.