My component where I am passing validate prop ->>>>

<div className="label-input-container activity-reason-dropdown">
                <Dropdown
                    dataAutomationId="identification-activity-reason"
                    name="identificationActivityReason"
                    label={<Translate id={isFromParameter ? 'updateParameter.updateReason' : 'commonComponentsLabels.activityReason'} />}
                    values={options}
                    disablePortal
                    disableClearable
                    mandatory
                    **validate={(value: any) => {
                        val(value);
                    }}**
                    customDropdownClass="activity-dropdown-heigh-issue"
                    placeholder="Select Reason"
                />
            </div>

validator intermediate function ->>>>

const val = (value: any) => requiredActivityReason(value, customProp);

actual validating function ->>>>

export const requiredActivityReason = (inputValue: any, customProp: any) => ((inputValue || typeof inputValue === 'number') ? undefined : 'Please select activity reason');