The ornamental design for a hook for beltbuckles, as shown and described.
1. Hook for belt buckles
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Top
1.5 : Bottom
1.6 : Left
1.7 : Right
1.8 : Reference
Reproduction 1.8 is a reference view showing the state in use of the claimed design and forms no part thereof; the parallel oblique lines of reproduction 1.8 are to represent the claimed design only and form no part thereof.