You may find some usage here https://github.com/epam/ketcher/tree/master/example.
And ketcher have an init method https://github.com/epam/ketcher/blob/7343fddd2c979c31fefdcba21e5df0687167a842/example/src/App.tsx#L99. You can call setStructure(ketcher)
after window.ketcher = ketcher
.
P.S. window.ketcher is required, otherwise paste(ctrl + v) will raise a error.