“Μόλις” always goes with the future form of the verb. "τελειώνω" is present tense.
The future form of “τελειώνω” is “θα τελειώσω”. However we the omit “θα” if it comes right after “μόλις”. That is, instead of saying "μόλις θα τελειώσω", we say "μόλις τελειώσω".
So the correct way to say it is:
μόλις τελειώσω θα έρθω
or
μόλις τελειώσω θα 'ρθω
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.