How to modify a cutom UI component

I am trying to edit a custom UI component (attached). I need to add an editable Text field side-by-side each Checkbox.

I though I could just open the custom component with the Designer. Then drag and drop the Edit Fields onto the GridLayout.

Unluckily regardless of the control I pick from the component library, I cabnnot drop it onto the GridLayout. It does not stick to it.

My question is:

Shall I add regular components to the GridLayout programmatically, that is changing the generated code?

Otherwise?

Thank you in advance

Maura