You have to ask. The savings plan is not an exchange thing. Savings plans are something from the broker himself, who somehow procures a fraction (probably with the agreement of the market maker). Normally on the stock exchange you buy in full. In the case of shares where there is already a savings plan, there was simply a demand and there was enough interest.
It has annoyed me for years that this is the case. So I regularly check to see if this has changed in the meantime. And what can I say? I finally set up my savings plan today ❤️😍