Hi @animartis ,
Do you want the textinput display the default value at first, if you click the textinput, clear the default value in the textinput?
Firstly, let me explain why your solution did not work.
It is not supported to change the text that the textinput displays by using this kind of formula:
TxtBxQty.Text = ""
You could only change the textinput's Default property to decide what text display in the textinput by default.
"TxtBxQty.Text" is used to represent the text in the textinput. It could be used as a "text" data.
Next, could you tell me when you click the textinput whether do you want to make the selected item's textinput blank or all the textinputs blank?
1、If you want to make all the textinputs blank when you click one textinput, the solution is very easy.
1)set the TxtBxQty's Default:
If(var,"",ThisItem.Current)
2)set the TxtBxQty's OnSelect:
Set(var,true)
Then, at the beginning the textinput display the Current field value. If you click one textinput, all the textinputs will become blank.

2、If you only want to make the selected item's textinput blank after you click the textinput, the solution will be a little complex.
Because the Textinput's Default property inside the gallery will affect all the textinputs in the gallery. That's also the reason why all the textinputs become blank in the first solution.
To avoid this problem, you need to use collection.
Here's my similar test for your reference:
1)set the screen's OnVisible:
ClearCollect(collection,fruit11111)
//fruit11111 is my list name, please replace with your data source name
create a collection with the data of your data source
2)set the gallery's Items:
collection
//display the collection in the gallery, not the data source
3)set the textinput's OnSelect:
Patch(collection,ThisItem,{Current:""})
//update the selected item's Current field to blank
please notice that this will not affect the data source's data
4)set the textinput's Default:
ThisItem.Current
//display the Current field in the collection
In this way, at the beginning the textinput display the original data of the data source, if you click one textinput, the textinput's displaying data will become blank.

Please notice that since in this situation, the gallery 's data source is the collection actually, so all the formulas related to this gallery need to make a little, especially those related to updating records.
For example: If you use "Gallery1.Selected", I suggest you change to this:
LookUp(your data source name,ID=Gallery1.Selected.ID)
"Gallery1.Selected" means the selected item in the collection.
This formula will return the related item in your data source.
Best regards,