a example on how an webui extention can get the prompt from the ui

sd_webui_prompt_ext_demo.py

from modules import scripts
import gradio as gr


class DemoExt(scripts.Script):
    def __init__(self):
        super().__init__()
        self.promp_elm = None
        self.promp_n_elm = None
        self.on_after_component_elem_id = [
            ('txt2img_prompt', self.save_prompt_box),
            ('txt2img_neg_prompt', self.save_prompt_box_np),
            ('img2img_prompt', self.save_prompt_box),
            ('img2img_neg_prompt', self.save_prompt_box_np),
        ]

    def title(self):
        return 'Prompt ext demo'

    def show(self, is_img2img):
        return scripts.AlwaysVisible

    def ui(self, is_img2img):
        button = gr.Button('get prompt and n_prompt and process')
        res_textbox = gr.Textbox('process result')
        button.click(
            fn=self.do_process,
            inputs=[self.promp_elm, self.promp_n_elm],
            outputs=[res_textbox]
        )

    def do_process(self, p, np):
        return f'the prompt is:\n{p}\n\nand the negative prompt is:\n{np}'

    def save_prompt_box(self, on_component):
        self.promp_elm = on_component.component

    def save_prompt_box_np(self, on_component):
        self.promp_n_elm = on_component.component