Resize after maximise

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C#

I am developing a windows form application. I have a mainform, formless form and button. the mainform starts in a normal state and when i click on the button on the mainform it opens the formless form called users in a panel on the mainform. The formless form is centered in the main form and when i maximise the mainform the formless form anchor to fill the mainform. This is working correctly but if i maximise the mainform before clicking on the button to open the formless form the formless form does not anchor to fill the main form. I need help on how to fix this so that when i maximise the mainform and click on the button to open the formless form it fill the mainform. This is the code for opening the formless form:

What I have tried:

currentForm = childForm;
childFormContainer.Size = currentForm.Size;
currentForm.TopLevel = false;
childFormContainer.Tag = currentForm;
childFormContainer.Controls.Add(currentForm);
currentForm.Anchor = AnchorStyles.Bottom | AnchorStyles.Top | AnchorStyles.Left | AnchorStyles.Right;
childFormContainer.Visible = true;
currentForm.Show();
currentForm.BringToFront();