how to Update cascading drop down field by code ??


I am using 4 cascade drop down filters in my list and it is working fine but I want one of the cascade drop down should be filled with the value of child (by default). How to make this . here I attached my issue print screen. can u find the attachment plz and give some idea to achieve this.
Green color cascade dropdown control should be defaulted with above red color cascade control selected value.

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