Can I use code to set up the form instead of using the graphic interface?

You can set up the user interface with imperative code and generally about everything you do in the editor you can do in code in Godot.

A little warning: it will be way less efficient to do everything with code for UI design. It makes iterating on the placement and look of things much slower, so I would recommend getting comfortable with working with the editor.

That said, here's an example of how you could set up three UI nodes with code: a Label in a VBoxContainer in a PanelContainer. It's also important to know how to do that for the cases where you need to create new UI nodes at runtime.

var panel_container = PanelContainer.new()
add_child(panel_container)
# This is equivalent of using the Layout menu in the editor toolbar
panel_container.set_anchors_and_margins_preset(Control.PRESET_WIDE)

var vbox_container = VBoxContainer.new()
panel_container.add_child(vbox_container)
vbox_container.set_anchors_and_margins_preset(Control.PRESET_WIDE)

var label = Label.new()
vbox_container.add_child(label)
label.text = "Hello World!"

You create the nodes and add them as children of one another, like how you would do in the editor.