In SharePoint Online, you can set up "Choice" fields, with the option of allowing "Fill-in" choices, which means the user can manually enter their own value if the value they want isn't in the default dropdown.
This feature works a treat in SharePoint Online's native UI, however, if you overlay a PowerApps form on top of your SharePoint list, it doesn't play ball! The Choice field displays in PowerApps correctly, but doesn't allow you to fill in your own values.
How do we sort this?
Easy enough, actually!
In your PowerApps canvas, simply:
Select the Data Card containing your Choice field.
Select for the "Update" property of the Data Card
Add in the following Condition:
(replacing the DataCardValue4 with your own Control Name of course!)
More info on how to do this is shown in the annotated screenshot below.
Enjoy your new Choice Field which allows Fill In Values via PowerApps ☺️