In the Prettier - Help documentation it says: "To run Prettier automatically when a file is changed, open the Settings/Preferences dialog Ctrl+Alt+S, go to , select the Run on save checkbox, and specify the pattern that defines the set of files to be reformatted every time such file is saved. "
I don't see the "Run on save" checkbox in my installation:
IntelliJ IDEA 2019.3.4 (Ultimate Edition)
Build #IU-193.6911.18, built on March 17, 2020
Runtime version: 11.0.6+8-b520.43 amd64
VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
GC: ParNew, ConcurrentMarkSweep
Non-Bundled Plugins: Dart, Karma, com.intellij.plugins.html.instantEditing, intellij.prettierJS, io.flutter, org.intellij.plugins.hcl, org.toml.lang