How to add more fields to figure function?

Hello @FelCar! Unfortunately, you cannot add a field to figure.

If the note you want to add must be after the figure itself, then your code is the right way to do this. you can simply wrap the note in a function called figure-note and call it after the figure.

#let figure-note(body) = pad(x: 0in, text(10pt)[Source: #body])
#figure(
    image("imagenes/panda.jpg", width: 60%),
    caption: [A picture of a panda]
)
#figure-note[trust me]