Apply changes when click on the element

[kimpastro]

Hello guys, need some help again.

Scenario:

Have two entities selected, and when I click in my plugin need to calculate the sum of both selected LenX and apply this value on the next entity that i'll click. Like image below:

My code until now:

model = Sketchup.active_model
selections = model.selection

lenx_total = 0.to_i
leny_total = 0.to_i
lenz_total = 0.to_i

if selections.empty?
  UI.messagebox('Select at least one component.')
else
    selections.each do |sel| 
      lenx_total = lenx_total + sel.definition.get_attribute('dynamic_attributes', '_lenx_nominal')
  end
end
 
# All this is for test only, where i'm stuck and trying to discover something
model.find_entity_by_id(18618).definition.set_attribute 'dynamic_attributes', '_lenx_nominal', lenx_total
new_def = model.find_entity_by_id(18618).definition
model.place_component new_def

Thanks for your time.