Problem:
You are customizing SharePoint Form using the PowerApps.
As you may have noticed, PowerApps Drop-Down menu (Combo box) does not support SharePoint Choice fields property for Allow ‘Fill-in’ choices by the default.
Solution:
In PowerApps, create a logic that stores another text field value to Drop-down menu value. That way you can store the users fill-in choices to the SharePoint List in PowerApps forms.
In my example I have created SharePoint List which contains the following fields:
- Title
- My Drop-down menu (choice field)
- values (choice 1, Choice 2, Other, please give your own value)
- HiddenTextField (Single line of text)
In PowerApps I have set If-statement to the HiddenTextField Visible-property.
If(DataCardValue2.Selected.Value ="Other, please give your own value", true, false)
So, when the user select value “Other, please give your own value” from the “My Drop-down menu”, HiddenTextField is shown in the form.
Then I have set If-statement to My Drop-down-menu (Combo box) Update-event.
If(DataCardValue2.Selected.Value="Other, please give your own value", {
'@odata.type': "#Microsoft.Azure.Connectors.SharePoint.SPListExpandedReference",
Id: 0,
Value: DataCardValue4.Text
},DataCardValue2.Selected) 
So, when the item is saved in SharePoint list, PowerApps form takes the value from the HiddenTextField and saves it to the My Drop-down menu as “Fill-in choice”.
Conclusions
That’s it. You can store users Fill-in choices to SharePoint list also in PowerApps forms.
Jere Huovinen 
How do I set it so the text input is stored in the column, but the value isn’t added to the drop-down choice column for the next submitter? I want to store the “Other” value, but not add it to the original drop-down choice column.
LikeLike
Hi Eric, I think that your requirement is quite hard to implement. Did you find a solution?
LikeLike
Yes I found a way to do it. First, I didn’t need a hidden field in SharePoint. I just added fill in box that is not connected to any field in SharePoint.
I changed the Update code in yours to set value to this: Value: Concatenate(“Other: “,TextInput1.Text)
Then in the “Items” value I added a filter to filter out “Other: ” text: Filter(Choices([@’Invoice Processing Requests’].Cost_x0020_Object),Not(“Other:” in Value))
It stores the value in the same SharePoint field but doesn’t show it in the drop-down for new items.
LikeLike
Hi Eric, good to hear that you found a solution. Nice job!
LikeLike
Any way to do this on a multi value select choice field? I think the formulas need to be different
LikeLike
Yes, this workaround is only for single value choice field. For multi value select choice field logic and formulas needs to be different.
LikeLike