Erik Kostson
Ansys Employee

Just apply a remote displacement on the the top and bottom faces which has behaviour rigid.
(So 2 remote displacements in total, one for the top face and one for the bottom)
Where you apply the force push down (say with 1 mm) with the degree of freedom in the direction of the applied force, and fix the other 5. On the fixed opposite end fix all 6 dof.
Look at the reaction which has the 1 mm down displacement in order to get the buckling force.
The buckling force is just the reaction from the static analysis times the eigenvalue obtained by the buckling solver. So say the reaction is 100 N and the eigenvalue for the first buckling mode is 2 then the buckling load is 2*100 N.
All the best


PS: If one wants a force instead of a down movement in the static, then create 2 remote points on these 2 faces (top and bottom), apply remote displacement to one (remote point) leaving the dof free (fix the other 5) in the direction of the force and then apply a remote force to that remote point. On the other side (fixed) just use a remote displacement with all dof fixed. See below for this approach: