I am programming a desktop software for my workouts and I have a main form, in which I have a page for “short-term planning” (pg_KurzfristigeTrainingplanung). From that page I call another form (frm_KurzfristigeTrainingsplanung), where I enter the dates and the workouts I want to train those days. With a button I want to “feed back” those entries to that very same page “short-term planning” (pg_KurzfristigeTrainingplanung) from which I called the form for the planning.
The values I entered in the form shall then be entered into the page and also “stay” there until I change them again (as well after closing the entire program).

How can I initiate or tell the program to refer back to that page?

private void btnEintragen_Click(object sender, EventArgs e)
{
    pg_KurzfristigeTrainingsplanung pgKF = pg_KurzfristigeTrainingsplanung;

    // Schleife von 1 bis 7
    for (int i = 1; i <= 7; i++)
    {
        if (i == 1)
        {
            // Spezialfall für i = 1: DateTimePicker und TextBox
            DateTimePicker dtpDatum1 = (DateTimePicker)this.Controls.Find("dtpDatum1", true).FirstOrDefault();
            Label lblDatum1 = (Label)pgKF.Controls.Find("lblDatum1", true).FirstOrDefault();

            if (dtpDatum1 != null)  // && lblDatum1 != null
            {
                lblDatum1.Text = dtpDatum1.Value.ToString("dd.MM.yyyy");
            }
        }
        else
        {
            // Standardfall für i = 2 bis 7: Labels
            Label lblDatumFormKF = (Label)this.Controls.Find("lblDatum" + i, true).FirstOrDefault();
            Label lblTrainingFormKF = (Label)this.Controls.Find("lblTraining" + i, true).FirstOrDefault();
            Label lblFocusFormKF = (Label)this.Controls.Find("lblFocus" + i, true).FirstOrDefault();

            Label lblDatumpgKF = (Label)pgKF.Controls.Find("lblDatum" + i, true).FirstOrDefault();
            Label lblTrainingpgKF = (Label)pgKF.Controls.Find("lblTraining" + i, true).FirstOrDefault();
            Label lblFocuspgKF = (Label)pgKF.Controls.Find("lblFocus" + i, true).FirstOrDefault();

            // Falls die Controls gefunden wurden, deren Text übertragen
            if (lblDatumFormKF != null && lblDatumpgKF != null)
            {
                lblDatumpgKF.Text = lblDatumFormKF.Text;
            }

            if (lblTrainingFormKF != null && lblTrainingpgKF != null)
            {
                lblTrainingpgKF.Text = lblTrainingFormKF.Text;
            }

            if (lblFocusFormKF != null && lblFocuspgKF != null)
            {
                lblFocuspgKF.Text = lblFocusFormKF.Text;
            }
        }
    }

    //Schliesst die Form nach erfolgreichem Eintragen der Daten
    this.Close();
}

I tried to feed back the values from the “planning-form” back to the earlier page.
I expected the values to show up, but the values did not show up on the page “pg_KurzfristigeTrainingsplanung”