The Magnetic Field node adds a boundary condition for specifying the tangential component of the magnetic field at the boundary:

The Harmonic Perturbation subnode (it is of the exclusive type) is available from the context menu (right-click the parent node) or in the Physics toolbar by clicking the Attributes menu and selecting Harmonic Perturbation. For more information see Harmonic Perturbation — Exclusive and Contributing Nodes.

Enter the value or expression for the Magnetic Field H0 (SI unit: A/m) vector coordinates. The defaults are 0 A/m.