Hi @Asghar Afzal ,
You need to create a new cutom dialog box extends the BaseDialog class, then you could use the pnp js to add items: https://pnp.github.io/pnpjs/sp/items/#add-items.
Here is an article shows how to create Custom Dialog Boxes: https://www.c-sharpcorner.com/article/spfx-creating-custom-dialog-boxes-without-a-javascript-framework/
Below is my demo for you: https://github.com/michaelhan-1/michael/blob/master/CustomDialog.ts
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

![93856-screenshot-5.png][3]