To clarify my earlier response, if you have one surface at 100°C, you actually expect to have a smooth thermal gradient such that the nodes adjacent to the 100°C surface may be 95°C (as an example), then nodes further away may be 90°C, etc. If you just specify 100°C on the nodes on the surface, the adjacent nodes will have your environment temperature, which is, by default, specified as 22°C. So over a very short span, your temperature drops from 100°C to 22°C!
Since we don't know how the part may cool down away from the 100°C surface (if that surface is really maintained at 100°C), that is why you should perform a heat transfer analysis first - the temperature field outside of the 100°C surface is unknown, so we solve for that first. Once we have determined the temperature field for your entire system/assembly, Mechanical can then automatically map the temperature field in your structural analysis, so you get correct & more realistic thermal strains, stress distribution, etc.
I hope that may clarify my earlier statements on why you probably don't want to just specify 100°C on a surface. This is actually this functionality is not directly in Mechanical, as it typically doesn't make sense to specify a vertex, edge, or surface of a part to have a different temperature than the rest of the body.