Let's imagine you first run a geometry optimization with the following input file
start h2o
geometry
0 0. 0. 0.
...
task dft optimize
Then, you can perform a frequency calculation with the optimized geometry of the previous run (provided that you kept the h2o.db and h2o.movecs files) by using the following input file that uses the restart option
restart h2o
task dft frequency
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