Сalculate the value of a field when form opened

​@bihalivan15, try this approach - add RowSelecting Event Handler in your graph extension, for instance:

 

public class BAccountMaintExt : PXGraphExtension<BAccountMaint>
{
    private bool _isCalculated = false;

    protected virtual void _(Events.RowSelecting<BAccount> e)
    {
        if (e.Row == null) return;

        // Check if we're in the primary view and haven't calculated yet
        if (!_isCalculated)
        {
            CalculateCustomField(e.Row);
            _isCalculated = true;
        }
    }

    private void CalculateCustomField(BAccount account)
    {
        // Your calculation logic here
        string calculatedValue = PerformComplexCalculation(account);

        // Set the unbound field value
        var ext = account.GetExtension<BAccountExt>();
        if (ext != null)
        {
            ext.CustomField = calculatedValue;
        }
    }

    private string PerformComplexCalculation(BAccount account)
    {
        // Replace with your actual calculation logic
        return $"Calculated value for {account.AcctName}";
    }
}