How to select general (unspecified) range that is active ?

Hello to everyone, how are you today ? I am little bit sad (or mad) because I am stuck at relatively easy task (but not for me now as I discovered) and cant find nothing useful on net , so please help:D

I have this macro for searching text in whole column and then add answer to next column - row. The problem is that the search range is only for specific column (now only for C1 to its end), but I want to search the text for any active column that I will select. So I need to know how to define, that the range will be dynamic or general for all columns that I select to search in, not only column C or some other. Hope I explain it clear.

Sub FindWord()

'ZADANIE DO SESDRU

    Dim FindString As String
    Dim FindCell As Range
     
    FindString = "Need to be added to SESDR"
    myreplace = "Add to SESDR"
    If FindString = "" Then Exit Sub
     
   For Each FindCell In Range(ActiveSheet.Range("C1"), ActiveSheet.Range("C65536").End(xlUp)).Cells
        If InStr(FindCell, FindString) > 0 Then FindCell.Offset(, 1) = myreplace
    Next FindCell