This is Small Post But Very Commonly Required.....
First and Fore most is to Declare Global variables for which fields we want to add Request Page fields ..
Let's get into situation in my case say i want to show Drill down to select a Customer No.
First define a Variable of same datatype as you may use in SETFILTER OR SETRANGE..
Just like below....
So in Customer---ONPREDATAITEM()
you can define SETFILTER(Customer."No.",'%1',Custfilter);
Now getting to Request Page...
Open Request Page And Give some Name To Container and select If you want Group /Field Then
SourceExp Open Globals by selecting Assist Edit Button And Select Your Defined Variable....
Now Select Your Field And Open Properties Window By Shift + F4..
And change Table Relation Select the respective Table And Field.... So it would take it as INPUT....