Well FileMaker doesn't come with a crystal ball, so somehow it would have to "know" that the donation is from Donor ID#3 and earmarked for Missionary ID #JD1 (and including the missionary's initials in their ID is not the optimal way to set up a primary key.)
Unless you have some sort of system that designates that all donations from John Smith are earmarked in a specific way, you'll need to select the missionary at the time you log the donor's donation.
The simplest method to set up (not necessarily the best method) is to set up the Missionary ID field with a value list of Missionary ID's and names. You select the missionary by name, but the value list enters the missionary ID to link that donation to that specific missionary. (And might one donation need to be "split" between two or more missionarys or projects?) For relatively small numbers of missionaries, this type of value list works well, but as the list to select from get's larger, there are more sophisticated options that require scripted support that make it easier to select the correct person from the list.
Your Donor ID field could be set up the same way, or you might use a portal to donations from a Donor table to enter donations and then there is no need for such a value list for the donor--it'll depend on your layout design.