(1) Defining Regions to Regions
region-to-regions    RegionsHeatSourceFeature-r2r-0 Lattice Lattice
Here Lattice is the name for the region where we apply heat source.
Here RegionsHeatSourceFeature-r2r-0 is the name for the region-to-regions
Add RegionsHeatSourceFeature for it
model-feature-r2r  ConjugatedFlowModel flow-set    reg-set-1  RegionsHeatSourceFeature RegionsHeatSourceFeature-r2r-0
or
model-feature-r2r  EnergyModel flow-set    reg-set-1  RegionsHeatSourceFeature RegionsHeatSourceFeature-r2r-0
(3) write the output file
write-sim        outputFixedT.stree
(4) Edit the outputFixedT.stree.
Search for RegionsHeatSourceFeature and set the heat source.