@markl42
Option 1:
Powerapps has out of the box functionality for a popup quick notification on top of the page; which disappears automatically. If you are interested, this would be the easiest way to go. Insert the below code on the OnChange property of the DropDown:
//Notification Pop Up on when OPtion "A" is selected from Dropdown
//OnChange property:
If(Dropdown1.SelectedText.Value = "A", Notify("Please complete a specific text box with justication",NotificationType.Warning))

Dopdown1 - is the name of the control in my app. You will probably need to replace is with DataCardValue1.Selected.Value
Option 2:
If you want to have a physically screen (object) that blurs everything out and forces your users to read your pop up. You can use this method.
In the OnChange Property of your dropdown, set a variable to "true" which will control the visibility of the your popup:
Set(varIsVisiblePopUp, true)
Set the Visible property of your Pop Up to this variable
//Visible property of Pop Up
varIsVisiblePopUp
If your popup has a cancel(close) button or icon, use the OnSelect property to toggle the varIsVisiblePopUp variable to false. This will then hide your popup.
//OnSelect property of Cancel/Close button or icon
Set(varIsVisiblePopUp, false)

Lastly, you will always want to hide the pop up when the user first visits the screen; so for good measure, I would default the varIsVisiblePopUp to false when the screen becomes visible. To do this, go to the OnVisible property of the screen and set varIsVisiblePopUp to false.
//OnVisible property of Screen
Set(varIsVisiblePopUp, false)
Either option should work. Good luck.
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