I am using the processing modeler in QGIS 3.10. I want to buffer polygons with individual field attributes.

This is not a problem when I am not using the processing modeller since one can simply choose the respective attribute field from the loaded layers. However, when I want to automize the workflow in the processing modeler, I don't know how to tell QGIS from where it should take the attributes since there are no preloaded layers in the processing modeller (see picture attached).

I guess, I have to use the "Expression string builder", but I don't know which command I should use. Let's assume layer x contains the field y, which has the respective attribute values I want to use for buffering (which differ by polygon, so it's not a constant value).

How do I express that in the "Expression string builder", when using the processing modeler?