To test your case I'd done the following in Windows 10 with Mathematica 12.3:

Use `Python` to generate 3 files, each with a different form (`\n`, `\r` and `\r\n`):
```python
with open('file1.txt','w',newline='') as f:
    f.write('sample\n')

with open('file2.txt','w',newline='') as f:
    f.write('sample\r')

with open('file3.txt','w',newline='') as f:
    f.write('sample\r\n')
```
I used Windows notepad and Notepad++, which both copied the exact same content that was written.

Also in Mathematica, if we read those file using `ReadString`, we'll see the original content:
```
ReadString["file1.txt"] // FullForm
(*Out: "sample\n" *)

ReadString["file2.txt"] // FullForm
(*Out: "sample\r" *)

ReadString["file3.txt"] // FullForm
(*Out: "sample\r\n" *)
```


After pasting in Mathematica, all forms (`\n`, `\r` and `\r\n`) becomes `\n`:
```
(* copied from file1.txt content*)
file1 = "sample
";

(* copied from file2.txt content*)
file2 = "sample
";

(* copied from file3.txt content*)
file3 = "sample
";


FullForm[file1]
(*Out: "sample\n" *)

FullForm[file2]
(*Out: "sample\n" *)

FullForm[file3]
(*Out: "sample\n" *)


file1 == file2 == file3
(*Out: True *)
```

### Solution
We can change the paste function in the menu bar to insert the raw format like `sample\r` instead of `sample` with a new line. If you're on Windows, open `MenuSetup.tr` in the `Mathematica_Directory\12.3\SystemFiles\FrontEnd\TextResources\Windows`.

If you're using other languages in Mathematica, go into the language folder for example for Spanish the file exists in `Mathematica_Directory\12.3\SystemFiles\FrontEnd\TextResources\Spanish\Windows`

If you're a Mac user, we face some problems in [this post][1]. If you find a workaround, please comment it, so all of us could enjoy it.

In the file, search for `paste` to reach this line (Windows and Mac seem to have little difference):
```
MenuItem["&Paste", FrontEnd`Paste[Automatic], MenuKey["v", Modifiers->{"Control"}]],
```
Add this code which was inspired by @kglr [post][2] after the above line to have two methods of pasting in Mathematica:

```
MenuItem["Paste 2", KernelExecute[NotebookApply[InputNotebook[],StringReplace[StringTake[RunProcess[{"powershell", "(get-clipboard -raw) -replace '\\r','\\r' -replace '\\n','\\n' -replace '\\t','\\t'"}, "StandardOutput"], {1, -3}], {RegularExpression["(?<!\\\\)\n"] -> "\\n", RegularExpression["(?<!\\\\)\r"] -> "\\r", RegularExpression["(?<!\\\\)\t"] -> "\\t"}]]],MenuEvaluator -> Automatic],
```
Save the file, restart the Mathematica and use the new `Paste 2`:

[![enter image description here][3]][3]

Because `Paste 2` runs a `PowerShell` code to get the clipboard content, it's a little bit slow.


  [1]: https://mathematica.stackexchange.com/a/249873/77079
  [2]: https://mathematica.stackexchange.com/a/199843/77079
  [3]: https://i.sstatic.net/WB1yo.gif