https://github.com/bitcoin/bitcoin/blob/5180a86c96bc05d2a731f70f36aae28ab5a3fad4/src/qt/sendcoinsdialog.cpp#L339

I think this is %1, not %2
{
    if (model && model->getAddressTableModel())
    {
        
        CoinControlDialog::coinControl->destChange = CNoDestination();
        ui->labelCoinControlChangeLabel->setStyleSheet("QLabel{color:red;}");
        const CTxDestination dest = DecodeDestination(text.toStdString());
        if (text.isEmpty()) 
        {
            ui->labelCoinControlChangeLabel->setText("");
        }
        else if (!IsValidDestination(dest)) 
        {
            ui->labelCoinControlChangeLabel->setText(tr("Warning: Invalid Bitcoin address"));
        }
        else 
        {
            if (!model->IsSpendable(dest)) {
                ui->labelCoinControlChangeLabel->setText(tr("Warning: Unknown change address"));
                
                QMessageBox::StandardButton btnRetVal = QMessageBox::question(this, tr("Confirm custom change address"), tr("The address you selected for change is not part of this wallet. Any or all funds in your wallet may be sent to this address. Are you sure?"),
                    QMessageBox::Yes | QMessageBox::Cancel, QMessageBox::Cancel);
                if(btnRetVal == QMessageBox::Yes)
                    CoinControlDialog::coinControl->destChange = dest;
                else
                {
                    ui->lineEditCoinControlChange->setText("");
                    ui->labelCoinControlChangeLabel->setStyleSheet("QLabel{color:black;}");
                    ui->labelCoinControlChangeLabel->setText("");
                }