As Daisy says the trick with these is warmth and sun - if that one is from the Netherlands it must have been grown in a hot green house. I use pumice iso perlite but my soil mix is pretty much the same as Daisy's.
They are happiest when night time low temperatures are over ~17 C - not sure if that is something you get a lot. If they get below ~5 C at night consistently, these plants will start going dormant (indicated by the plants dropping their leaves) if you water them in these cold periods during their dormancy you will more likely than not rot and kill it. They are very touchy to cold and wet, more so than (m)any cacti.
Many of these (though I am not sure about your thin leaved version) are from regions that are hot and pretty dry but often with a surprising amount of rainfall, so when it is warm at night and hot during the day they actually can be watered way more than your average cactus, but I am not sure you would get hot enough during the day, say consistently >30 C for days/weeks at a time with hot bright sun on them, so I'd be careful with watering no matter what. Also make sure it does not end up sitting in any kind of standing water for any length of time.
So treat it with a lot of care, get it in the brightest light possible (but keep in mind that it probably grew in a green house, so even not very strong direct sun could still scorch/burn it) and then monitor soil wetness and night time temperatures and water with care.
Mark Dimmitt who is a long time cultivator and hybridizer of these based in Tucson, AZ, writes in his book that in the low desert of Arizona, for most of the year you can water Adeniums like you would a tropical plant in terms of watering frequency rather than a cactus, as long as the soil is well draining and quick drying. Once I started doing that I got my Adeniums to flower more consistently and longer, but we get lows >17 C at night for a large part of the year, when it is between 5 and 17 C at night I water them maybe once every 2 weeks, then when it gets below 5 C at night consistently (which does not always happen here) I stop watering them completely. This winter half of mine went dormant and the others which were located deeper under patio and closer to the house did not and they kept their leaves through the winter. We started getting consistently over 30 C for day time highs somewhere in late March and will not go down below it much or often until October/November, so I have been watering my plants quite heavily.
Here is one of mine with similar but not the same rather thin leaves that is currently putting on a flower show. It is an Adenium crispum hybrid that I got from Dimmitt when I attended one of his lectures on Adenium cultivation, not much of a caudex on this one, but since I upped the watering and regularly fertilize it with quarter strength orchid fertilizer it has become a much more consistent and floriferous bloomer:
Interestingly when it gets >45 C for a day time high on a consistent basis (which unfortunately happens here quite a bit as well in June - July), that actually turns out to be too hot for some of the Adeniums. I have an Adenium soccotranum that consistently goes dormant every summer and drops all its leaves. The crispum hybrid suffers through those periods, while my A. arabicum and my A. obesum hybrids take it with very little problem. Except that they all stop flowering.
OK, that last part about conditions here in Arizona are maybe not quite as useful for you, but it may provide some perspective on what these plants like.