It would look the same and be administered in the same way, but would have no drug in it. I imagine it would be a pretty coloured tab much like the real thing.

Unfortunately, even the full text of the paper doesn't say what it was or how it was administered.

https://www.nature.com/articles/s41598-017-17546-0

Edit: I just missed it - the details are: healthy participants (mean age 30.5 ± 8, 4 females) received 75 μg LSD in 10 mL saline (intravenous, I.V.) or placebo (10 mL saline, I.V.), 70 minutes prior to fMRI scanning. LSD and placebo sessions were separated by 14 days and occurred in a counter-balanced order

Not sure about this one, but in other studies they’ve used drugs like methylphenidate.