After defining your ladder stair in the position and geometry it should be in your project, activate the Define Set Stair Support Boundary Conditions command in the Ladder Stair auxiliary toolbar.
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In the dialog that opens, after marking the solution title with the Structure, make your arrangement so that only the option/options that are supported on each side of the staircase will remain active. If you want to solve the staircase element with the structure, it would be a better solution to select the Semi-rigid diaphragm option from the Analysis Settings/General Settings tab.