The short answer

Only a tiny amount of plain yogurt, occasionally.

Why

Birds are lactose-intolerant, but plain yogurt is low in lactose (the cultures break much of it down), so a tiny amount usually causes no problem. Avoid flavoured or sweetened yogurts.

How to feed it

If you offer it, a small dab of plain, unsweetened yogurt on a spoon or on some veg.

How much & how often

A tiny amount, rarely.