Enter the name of any model structure in the Initial model: edit box of the Parametric Models dialog and then press the Estimate button. The method is a standard prediction error/maximum likelihood approach that iteratively searches for the minimum of a criterion. Options that govern this search are accessed via the Options button in the Iteration control... dialog.
The name of the model structure must be a variable in either the MATLAB workspace or on the Model board. In the latter case, you can just drag-and-drop the model icon over the Initial model: edit box.
Arbitrary model structures can be defined using several commands in the System Identification Toolbox:
(file iduibn.htm)