Setting Focus on an instance of windows form

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C#

WinForms

I have a C# program.
when you push a button,I want it to load another windows form that has different tabs.I want it to start from different tabs,when I push different buttons.
for example:
I push the: 'student' button,I expect it to open the windows form with student tab selected.
and when I push the: 'teacher' button, I expect it to open the same windows form with teacher tab selected.

I use this code to open windows form:

Form frm = new Form2();
frm.ShowDialog() // or even Show(). but neither worked :(

I have used select(),and focus() methods.but they didn't work :(
and also I used this approach:

ActiveControl = frm.Controls["tbTabs"].Controls["studentTab"];

but it didn't work either :(