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I have several comboboxes with the small dropdown grey box on the left that needs to be clicked to drop down the choices. I want to create a transparent button that covers the entire box so that when you click on it anywhere, then it does the same as clicking on the grey drop down arrow.

No need to go to all that effort.  Just use the .DropDown property for the Combobox in the On Got Focus event.

    NameOfYourCombo.Dropdown


Covered in Access Developer 2

Please explain how to get to and use  .DropDown property for the Combobox in the On Got Focus event.

I just did.  

In design view click on properties.  Events tab, On Got Focus.  Click the ...
Choose Code builder.

Type in the name of your ComboBox.Dropdown

Save - Close and reopen.

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Do yourself a favor and learn to search on this site. You might not know exactly what you need but the search is pretty comprehensive.

I have tried that by using the following on the OnFocus property
Private Sub MedicineID_GotFocus()
Me!MedicineID.Dropdown
End Sub

I then saved, closed and re-opened the database. Now when I go to that field it opens a blank dropdown. When I click the arrow normally it brings up the first dropdown choice.  Once I added this routing I get a blank dropdown.

Is the Combo Box name actually MedicineID?

In the picture CustomerID is the Data but CustomerCombo is the name of the control.