sync data on data field from key field in the sub form

[iccsi]

I have a sub form which is on datasheet format.

There are 2 fields are hand in hand and I would like to sync 2 fields
in the datasheet sub form.
For example,

I have KeyField, DataField in the sub form and would like to have user
select KeyFiled and fill Data Field base on user select.

I use txtDataField.value = cmbKeyField.column(2).value, but for some
reason it does not fill the text box of DataField control,

Any help is great appreciated,