https://analysistabs.com/vba/append-existing-text-file-excel-macros-examples-codes/

Sub VBA_to_append_existing_text_file()
    Dim strFile_Path As String
    strFile_Path = "C:\temp\test.txt" ‘Change as per your test folder and exiting file path to append it.
    Open strFile_Path For Append As #1
    Write #1, "This is my sample text"
    Close #1
End Sub