You have to make it before adding sugar or honey or condensed milk, and secondly, you must use plain milk or it won't ferment.

When making yogurt, you can add sugar or honey or something like that, that is called old yogurt. But the added sugar or honey must be sterilized, otherwise there is a possibility of pouring to make yogurt failure. The reason is that the yogurt machine must be sterile. If the honey is not sterilized, it may affect the fermentation of the yogurt. The milk used to make the strain must be bought as fermented, not as a milk drink.

Extended information;

The production process of yogurt can be summarized as ingredients, preheating, homogenization, sterilization, cooling, inoculation, (filling: used for curdled yogurt), fermentation, cooling, packaging and post-cooking of several processes, modified starch is added in the ingredient stage, and the application of its good or bad effect has a close relationship with the control of the process:

Batching: Select the required raw materials according to the material balance table, such as fresh milk, sugar and stabilizer. Modified starch can be added separately at the time of batching or dry-mixed with other food gums and then added. Considering that starch and food gums are mostly hydrophilic macromolecules, it is better to mix them with appropriate amount of sugar and dissolve them in hot milk (55℃～65℃, the selection of the specific temperature depends on the instructions of modified starch) under high speed stirring to improve its dispersibility.

Source:Baidu Encyclopedia-Yogurt