As a first approximation, you can use a constant head node with the approximate well level that you target. FEFLOW gives you the balance on the node, corrresponding to the pumping rate. There are, however, a few more conditions to consider: The balance depends on spatial discretization, so you should use the so-called ideal element size around the well if you want to get the budget right (depending on the radius of the planned well). There is also an optimum water level in the well that gives you maximum inflow (in case your aquifer is unconfined) as with too much drawdown, the inflow reduces due to reduced transmissivity while with not enough drawdown, the low head gradient limits the inflow. FEFLOW will not calculate that ideal drawdown for you, but you can estimate it in advance from analytical equations or simply do some tests with different levels in FEFLOW.
