USER CAN GIVE INPUT MANUALLY INTO LOOKUP FIELD IN AX

TASK:

In my Table I have serial number field in that I want to enter manually serial numbers not taken from lookup but at the same time i don't want to save in InventSerial Table but i need to save my input into current table.

Simple Steps:
In Table level methods,

public boolean validateField(fieldId _fieldIdToCheck)
{
    boolean ret;
    ;
    if (_fieldIdToCheck == fieldnum(CurrentTable,InventSerialId))
        ret = true;
    else
        ret = super(_fieldIdToCheck);
    return ret;
}