A value of control property is written to the associated OPC item when a specified event is triggered. For example, you can link the Write to the button click.

How to configure this feature:

On a TextBox (or similar control), use "Bind to Point" command and select the OPC-DA item (or OPC-UA node) you want to write to.

 Then, use the "Edit Live Bindings" command, and for the BindingOperations options, select only "Write". In order to have the Write invoked when the button is pressed, set the WriteEventSource.SourceComponent to the Button control; the SourceMember of the WriteEventSource will be automatically set to the Click event. This causes the Click on the Button execute the Write operation, i.e. obtain the value from the control and write it into the OPC item.