Input only one character of the selecting first syllable

[tats-u]

ATOK can do the following operations on the selecting first syllable:

Confirm the first character of it by pressing Shift + ↓:

atok-confirm-1st-character.mp4

https://atok.com/other/support/howtouse/mac/tr/pgs/tr_conv_one_josi.htm

Input only the first or last character of it and remove the rest of it by PageDown (first) or PageUp (last) (ATOK style key preset only):

atok-only-one-character.mp4

https://atok.com/other/support/howtouse/mac/tr/pgs/tr_conv_one_bunsetu.htm