I’d write a bookmarklet for that case:

javascript:
{
const name = 'ABC';
const d = new Date();
const year = d.getFullYear();
const month = d.getMonth();
const date = d.getDate();
document.activeElement.value = `${year}/${month}/${date} ${name}`;
void 0;
}

This bookmarklet inserts the desired text into the currently focused text box. Tested on Lemmy Web UI.

What program are you using to write or edit the comments?