For example lets take an example of BAPI_PO_CREATE1.
1) Find the package of the BAPI in which it is stored.
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2) Goto SE11 and give BAPI_TE_* in the data type and press F4.
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3) Click on structures
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4) You will get a huge list of structures. click on new selection
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5) In the pop up provide the package name from step 1 and press OK.
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6) Now you will get a list of structures related to that BAPI in step1.
