As you want to perform the P-Delta analysis, we suggest you to use the REPEAT LOAD instead of the LOAD COMBINATION in order to obtain the actual second order effect.
One of the syntax as shown below combines the lateral load and the gravity load
*****************************************************
LOAD 11 LOADTYPE None TITLE LOAD CASE 11
REPEAT LOAD
9 1.0 1 1.0
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Also to invoke the P-Delta analysis, you can simple add the PDELTA ANALYSIS command replacing the existing PERFORM ANALYSIS command.
However, you can also fine tune the P-delta analysis like ignore the small delta or you can simply use the KG matrix option to run the P-delta analysis.
Please click on the link below that discusses more on this topic.
communities.bentley.com/.../22312.p-delta-analysis-with-verification