Add control transparent in a Panel

I assume you talk about System.Windows.Forms. Short answer is: switch to WPF. In certain problems, Forms presents serious problems with transparency, and, where those problems exist, they are not going to become easier. Transparency at the level of control is the case.

Alternatively, you can use Forms with your own rendering not at the level of controls, but within one control, using System.Drawing.Graphics directly, in the handler of the event Paint or by overriding the virtual method OnPaint. At this level, transparency is not a problem at all. Please see also my past answers:
Drawing Lines between mdi child forms[^],
capture the drawing on a panel[^],
What kind of playful method is Paint? (DataGridViewImageCell.Paint(...))[^].

—SA