The md.basalforcings.groundedice_melting_rate and md.basalforcings.floatingice_melting_rate are fields are used in the mass transport equation to specify how much ice melts at the base of the ice for the grounded and floating parts, respectivetly. The thermal model computes the amount of basal melt at the base for the grounded ice, and these values will be used if you run a transient model instead of the ones specified in me.basalforcings.groundedice_melting_rate in that case.
To force the basal temperature you can do it in a similar way that is done for the surface temperature using md.thermal.spctemperature.