|Here is some "basic VBA".|
Some assumptions had to made since you didn't supply a lot of detail.
The Drop Down is in Sheet1!A1
The destination for the ID number pulled by the macro is Sheet2!A2
This code will monitor changes on Sheet1 and if a change is made to the Drop Down (A1) it will fire. (There's no need for a "run macro" button")
It will look in Sheet3!B:B for the value chosen in the drop down. When it finds the value, it will place the ID number from the corresponding row on Sheet3!A:A into Sheet2!A2.
Right-click the Sheet tab for Sheet1, chose View Code and paste the macro into the pane that opens.
Private Sub Worksheet_Change(ByVal Target As Range)
'Determine if change was made to drop down cell
If Target.Address = "$A$1" Then
'Find drop down value in Sheet3!B:B
Set p = .Find(Sheets(1).Range("A1"))
'Return corresponding value from Sheet3!A:A to Sheet2!A2
Sheets(2).Range("A2") = Sheets(3).Range("A" & p.Row)
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