Hi Yoon,
To apply a pressure over a segment of your model, you can first define a segment using *SET_SEGMENT and then apply pressure over the prescribed segment using *LOAD_SEGMENT_SET. Please check the details of these keywords in the keyword manual:
https://www.dynasupport.com/manuals/ls-dyna-manuals/ls-dyna_manual_volume_i_r13.pdf
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