How to show all combobox items when on focus

See more:

C#

Windows

combobox

application

I am making a windows application in which interaction is 100% keyboard so the user moves around widgets and options using tab.
When user enters this page, the focus is on textbox, so he can write there and if he tabs, he goes to the combobox which is showing item1 by default. If he presses down arrow, combobox shows item2. The problem here is that with this way the user don't know all the options of the combobox unless he press down until the last option.
What I want is when user enters, the focus is on textbox . BUT then, when he tabs and goes to combobox, I want this combobox to show all options (as when you click with the mouse, that you can see all the options in a list).

What I have tried:

I have been looking for dropdown options and views but couldn't find one that makes that... do you know if there is any option to do that?