linear buckling with lateral loads

I built a simple model to test advanced solver linear buckling.  I found that adding a lateral load at mid-span (in combination with a compression load at the top of the member) does not affect the buckling factor.  Is there a way to modify the model to include the affect of the midspan load?

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 11-Apr-18
END JOB INFORMATION
INPUT WIDTH 79
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 0 20 0; 3 0 10 0;
MEMBER INCIDENCES
1 1 3; 2 3 2;
DEFINE MATERIAL START
ISOTROPIC STEEL
E 4.176e+006
POISSON 0.3
DENSITY 0.489024
ALPHA 6e-006
DAMP 0.03
TYPE STEEL
STRENGTH FY 5184 FU 8352 RY 1.5 RT 1.2
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
1 2 TABLE ST W6X15
CONSTANTS
MATERIAL STEEL ALL
SUPPORTS
1 FIXED BUT MX MZ
2 FIXED BUT FY MX MY MZ
LOAD 1 LOADTYPE None TITLE LOAD CASE 1
JOINT LOAD
2 FY -10
JOINT LOAD
3 FZ 1
PERFORM BUCKLING ANALYSIS
FINISH