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.
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])