1) Here's the code for in Power Apps on the button.
ClearCollect(myemails,ComboBox1.SelectedItems);
SendEmail.Run(JSON(myemails,JSONFormat.Compact),User().Email);
Create a collection from the SelectedItems in the Combobox. Then use JSON() to translate that collection into a JSON formatted array as a parameter.
2) Then in the flow. Add a Parse Json action to parse the collection array you send as a parameter. Since its an array you'll need an apply to each to get the emails into a semicolon delimited string variable. Use that variable for the To:
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