How to handle WRITE Function in Pi with STRING "Point Type"

Can anybody tell how to handle WRITE functionality using STRING Point Type in OSI Pi?

We are expecting that when there is change in source Tag (STRING Point Type) ; instrument Tag ((STRING Point Type) should get value from source Tag.

Is it mandatory to pass SOURCE TAG while handling write function? OR can we leave source TAG empty. At present using SDK we are trying to write using Point Name (Source TAG Empty). But STRING is not working, intermittently it is causing Interface service to fail.

For additional information write functionality is working fine for integer type (Both Source and Instrument TAG) but same functionality is not working with STRING tag.