The rover always works in the same datum the base has. So if the provider works in ISN2016 or ISN93, you’ll need to choose the corresponding coordinate system based on the same datum. Otherwise, you’ll get a conversion error that may lead to a possible mismatch of coordinates.
You don’t need to send any coordinate system to the provider and Emlid Flow also doesn’t do this. The process looks as follows:
The base sends its geographical coordinates to the rover
The rover calculates its geographical coordinates regarding base coordinates
The app performs conversion from geographical coordinates to local coordinates using CS parameters
So the coordinate system parameters should be defined beforehand while creating the project.