We first choose Custom Field Function in the Define  menu.
 
 

Then appears the custom field function calculator where we can define our function. In our case, we only used functions that are in the grid menu, namely the x-coordinate and the y-coordinate.
Lots of field functions are available, such as pressure, velocity, fluid properties and velocity derivatives.
 

Once the custom field function is defined, we need to apply it to the right zone.
With the example of a function called temp, that is suppose to give a gradient of temperature as the initial condition:

To apply the function, we have to use the application patch in the sub menu initialize of the solve menu.

Once this application is chosen, we patch the variable using the custom field function like in the following.