The total influx can be prescribed as a point source using the well-BC (4th kind BC), unit m3/d.
To assign the total inflow along a line (e.g., a border section), you need to calculate the length-specific inflow in m2/d from the length of the section (m) and the total flux (m3/d).
In a 2D model, you can assign this value as a depth integrated boundary condition (m2/d) (in case of an unconfined model, you have to choose the integrated option explicetly).
In a 3D model, you have to calculate the inflowing darcy velocity (m/d) from the length of the section and the depth of the model.