Tremolo is added in Write mode using the buttons in the repeats panel on the right-hand side (we include tremolos in the repeats panel because they are, at least in some idioms of music, considered a shorthand for repeated notes, just as more broadly repeat structures are a shorthand for repeated sections of notes). However, as you may have found, you can only add them to all the notes in a tie-chain in Write mode, because you can only select the entire note in Write mode (which reinforces that it’s really a single note under the hood).
When you need to show the tremolo on just one note in the chain of ties, then you do indeed need to do this in Engrave mode via the Properties panel.
At the moment, these do not play back with the tremolo effect on the right notes, but this is in our future plans.