In a structural analysis, you should be careful since the nodes that do not have temperature assigned will default to having the environment temperature assigned to them. This means that, if you are performing a purely structural analysis and have 100°C applied only to a surface, the rest of the body may be at 22°C. This is unrealistic and can cause artificially high stresses (in addition to wrong results).
If you expect a thermal gradient such that one surface is maintained at 100°C while the rest of the body may have heat dissipated, I strongly suggest that you perform a thermal analysis first. You can link the temperature results to a structural analysis, so the imposed temperatures will be more realistic. Please take a look at this course for more info on performing thermal analyses as well as linking results from a thermal analysis to structural analysis:
Topics in Heat Transfer Analyses - ANSYS Innovation Courses