how to make condition using string

i need your help, i just learned about c# and i need to do this for my project. i need to make my textbox become auto fill, so if i choose one of the option from combobox then click button save (or not, so after choose it would automatically fill with text), the other textbox will be auto fill without have to type manually. thanks!

    private void btnSave_Click(object sender, EventArgs e)
    {
        if (cmbBranch.SelectedText == "NewYork")
        {
            txtBranchNumber.Text = "1";
        }