Spin-polarized electrons injected from a ferromagnet (FM) onto the surface of a topological insulator (TI) tend to produce a charge current transverse to the direction of the spin polarization because of the spin-momentum helical locking of the TI surface states. The charge current can be measured as an open-circuit voltage that will change in sign if the magnetization direction of the FM is reversed. Here, we model the two-dimensional transport on the TI surface coupled to a FM through a tunnel barrier (TB). The transport equations are solved analytically for two different boundary conditions on the TI surface, and the effectiveness of the TI-TB-FM junction for determining such voltage change upon FM magnetization reversal has been derived for different device dimensions. Such measurement can be used to study the spin-momentum helical locking of the TI surface states as well as for reading the FM magnetization direction in memory and logic devices based on TI-TB-FM heterostructures.