The output of the surface shader needs to be connected to asClosure2Surface (see screenshot below). This is similar to Cycles where the surface shader nodes are connected to the Material Node.
You can connect the Transparency output to the asClosure2Surface instead of the Color output of asStandardSurface but that is not necessary. A change in transparency affects also the Color output.
appleseed studio is not really meant for scene creation or extensive modification. It is more a scene debugging and inspection environment. You cannot user friendly create a scene/materials from scratch with it.
When you export scenes from e.g. blenderseed then appleseed materials get translated into shader groups (see extract from scene file below). Unfortunately these shader groups are not (yet) editable with the Attribute Editor in studio.
I would suggest using the plugins for scene editing and then export to studio if you need to tweak.
Extract from exported scene file:
<shader type="shader" name="as_standard_surface" layer="asStandardSurface">
<parameter name="in_color" value="color 0 0 0" />
<parameter name="in_face_tint" value="color 0.258724 0.316538 0.396061" />
<parameter name="in_ior" value="float 1.49" />
<parameter name="in_transparency" value="color 0.710384 0.557016 0.595661" />
<shader type="surface" name="as_closure2surface" layer="asClosure2Surface">
<connect_shaders src_layer="asStandardSurface" src_param="out_outColor" dst_layer="asClosure2Surface" dst_param="in_input" />