In this tutorial I will show a simple python script for add a String interface to the Group Output.
You can see in the next image the result of this script.
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You need to have an modifier Geometry Nodes or add new one.
This is the source script with comments for each step I used:
import bpy
#get active object - default
obj = bpy.context.active_object
# set the default start for working with Geometry Nodes modifier
# you need to have a Geometry Nodes modifier
node_group = obj.modifiers['GeometryNodes'].node_group
nodes = node_group.nodes
#get the node named 'Group Output'
geom_out = nodes.get('Group Output')
#create a string node
string_node = nodes.new('FunctionNodeInputString')
# set the name to 'String'
string_out = string_node.outputs['String']
# set the value to "This is a string"
string_node.string = "This is a string"
# link to the Group Output
node_group.links.new(string_out, geom_out.inputs[-1])