While looking for a list of phonemes in SynthV, I found out about glottal stops from an unofficial manual:

https://manual.synthv.info/quickstart/entering-lyrics/#adding-glottal-stops

Glottal stops can be added between notes by prefixing the following lyric with a single quote ('). A note containing just a single quote can be used to add a glottal stop at the end of a phrase.

[…]

While a “glottal stop” is, strictly speaking, the sudden termination of airflow (and the resulting sound), the same symbol can be used in Synthesizer V Studio to create hard onsets, by using the single quote (') notation at the start of a phrase.

However, the Synthesizer V Studio Phoneme Reference lists the glottal stop as using the phoneme cl.