KDB+ / Q unique random values with variable for amount

I am quite new to KDB+ and have a question about generating random numbers.

Lets say I want to create num random unique numbers.

When i use this

10?10
-10?10

I get 10 random numbers in line 1 and 10 unique random numbers in line 2 (range from 0 to 9)

When I want to introduce a variable like this

num:10
num?10  / works
-num?10 / dont work

The generation of unique randoms does not work.

Whats the correct syntax for this?

Thanks in advance