Hi Bina, currently we don't have a good way to get the checked value with the form submit.
You can listen the select event and store the checked values every time user click the checkboxes

const dt = document.querySelector("your-datatable-selector");
dt.onselect = function(e) { 
    console.log(e.detail.id, e.detail.selected);
		// do more stuff like store the values then pass the combined values into the form 
}

Also you can still get the values using javascript then you can pass the values with your own fields or hidden fields before submit happens.

const dt = document.querySelector("your-datatable-selector");
const ids = dt.rows.filter(row => dt.shadowRoot.querySelector(`#row-${row.id} q2-checkbox[checked]`)).map(row => row.id);

We'll open a ticket to provide a better way to get the checked values with the self.form_fields in the extension.py
Let me know if you have any questions.

Thanks
Alex