If you set the FMUName property then you should not get prompted. YOu can do this either by adding additional arguments to the add_block() function or call set_param after add_block as shown below.
fmuBlock = add_block('simulink_extras/FMU Import/FMU','untitled/FMU','FMUName','myFmu.fmu');
or
set_param(fmuBlock,'FMUName','myFum.fmu');