I came here to ask the same thing. You could do it on the reddit boost app.... appears to be missing on this one.
this post was submitted on 03 Oct 2023
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Boost For Lemmy
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If I remember what Ruben commented once. It was an option on Reddit's backend(or something, I don't know anything about website design) but it's not an option on Lemmy's.
I think it's something Lemmy will have to implement before the option can be added to Boost.