You can use the Function Tool to apply complicated boundary conditions on a model. The Function Tool has two parts: the Function Editor creates an arbitrary equation or function (multiple equations), and a Function Loader retrieves the functions and loads them as TABLE arrays. They are then applied to a model using the tabular boundary condition process described in Applying Loads Using TABLE Type Array Parameters.
You must define your function and load it as a TABLE array before you try to add it as a load. You can access the Function Editor either via Utility Menu> Parameters> Functions> Define/Edit, or via Main Menu> Solution> Define Loads> Apply> Functions> Define/Edit. You can access the Function Loader either via Utility Menu> Parameters> Functions> Read from file, or via Main Menu> Solution> Define Loads> Apply> Functions> Read file