[Freesurfer] First-level analysis with continuous covariate

How does one execute a first-level (subject-level) analysis with a continuous covariate?

For example, in an event-related experiment presenting pictures of scenes, I have two conditions: environment (indoors/outdoors) and openness (a continuous rating obtained independently). I am interested in the environment x openness interaction, so I need to model them together.

The parametric modulation paradigm file seems close to what I need, but in the example, there is no discrete variable to consider. Is there a way to model both discrete and continuous variables together?

If so, can this method also be used to model out nuisance regressors, like trial performance?

Thank you,

Emily