C# 3.5 winforms textbox display a watermark after the cursor while typing

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WinForms

C#3.5

watermark

Hi,

I have a Dot.Net 3.5 winforms control with a TextBox control.
This TextBox accepts uppercase letters and numbers.
If a value is entered, the currently maximum value should be displayed as a watermark behind the cursor.

Example: Input "A1C" and the maximum value is "A1CDB1", then "A1C" should be displayed normally, then the cursor and then "DB1" as watermark.

I know there are some examples that show a watermark when the TextBox is empty, there are also many autocomplete examples, but that's not what I need.

If the user enters "A1CDB1" or another existing value, the color of the input text changes to red.

What I have tried:

I tried to draw the watermark text behind the cursor position on OnPaint, but that was not successful.

Does anyone have a working idea/solution?