Attach Image to Post from both IDs

Hello, I’m using the plugin Alchimist Ajax Upload (https://wordpress.org/plugins/alchemist-ajax-upload/) to upload an image via Ajax, the post is created after submission of the form and after uploading the unattached images. My problem is that I need to attach those uploaded images to that created post. I have the post ID and the attachment ID, is there a php method that I could write to attach those two using just their IDs?
Thanks for any response I can get.