Hello community,
how can I modify a entity form in a popup?
I have an entity with a subgrid with data from related entity.
In subgrid is a create button to create related entry. By clicking this buttton always opens a popup with entity form.
But how to add a button into this popup form?
I know how to add a button into webpage, but in such popups?
Here is a picture of my situation:

Yellow in the background left: Main entity.
Blue in background left the subgrid of related entity records.
Violett in background right the clicked create-button.
Finally orange the place of new button in popup.
Thanks,
AndKan.