You can use the DSB, direct link to seat reservation (international): https://travel.b-europe.com/dsb-rail/en/reservation-only

Price is 30DKK per train, around €4.

Its sold out on this site

At what time you want to depart from Hamburg?

cd.cz also sells these reservations for around €3 but if it's sold out, then it's sold out.

Plan B is to look for journeys via Flensburg (no reservations required). Plan C is via Niebüll.

14:58 from hamburg altona 

All trains are available that day so please double-check.

And indeed book via cd.cz, it’s cheaper.

EDIT : It seems that the 14:58 train is sold out. Would it be possible to take the 12:58 train instead ?

I’m sorry but i don’t know what i’m doing wrong. 
I’m on cd.cz website and “the requested service is sold out on this train."

I think its possible

Avez vous bien renseigné le fait que vous aviez un pass Interrail, car je viens de regarder par exemple pour un départ à 10 heures et il n y avait pas d obligation de réserver... 

There is no departure from Hamburg Altona at 10 on 21 July. There's one at 8:58 (change at Fredericia) and at 10:58, both with mandatory  reservations. So where did you see a departure at 10 without mandatory reservations?

10h58...pas 10h00

OK, now look in the train details and you'll see an R in a square, and a remark at the bottom about mandatory reservation (in Czech).