If you have already created a requisition and want to associate it with an existing Mission, you can follow these steps:
1) Go to Orders > Missions.
2) Click on the actions menu of the Mission line that has the relevant expense, followed by View.
3) On top of the facebox will be the relevant line ID. In this image below is the number 918.
4) Go to Orders > Orders list.
5) Find the relevant requisition and click on the actions menu of the specific requisition line you wish to link to your Mission, followed by View line.
6) A facebox will appear. Go to the Related docs. tab.
7) Fill with the information with the number you identified above from the Mission line and click Add related document.
8) The requisition is now linked to the Mission line, as you can see by the airplane icon that appears () in the requisition line.
9) The Mission line will also have an icon () indicating that the line is linked to a requisition.