You're probably assigning the text field - which is an object with many different properties - to the variable TxtstadiumName by one of two methods. Either
Sub YourMacro(oEvent As Object)
oSubForm = oEvent.source.model.parent
TxtstadiumName = oSubForm.getByName("TxtstadiumName")
Or
Sub YourMacro
oSubForm = ThisComponent.drawpage.forms.MainForm
REM Make sure to change 'MainForm' to be your actual subform name
TxtstadiumName = oSubForm.getByName("TxtstadiumName")
Note that while Basic variables don't care about upper or lower case, the API method .getByName is case sensitive! The question has txtstadiumName in one place and TxtstadiumName in another, and it's important that whichever one is correct is the one in quotes inside the .getByName method.
Once you have the text field object assigned to a variable, now you want to change one of its properties. In this particular case I believe the property name is text
. So like this:
TxtstadiumName.text = ""