See if this works :
Code:
Option Explicit
Sub Einfügen_New_A_and_Fill_with_Content_SPECIFIC_SHEET()
Dim Data As String
Dim Result As String
Result = InputBox("Provide a sheet name.")
ThisWorkbook.Sheets(Result).Activate
Data = InputBox("What would you like to insert in column A ?")
ActiveSheet.Cells(1).Resize(ActiveSheet.UsedRange.Rows.Count) = Data
End Sub