In Mendix 4, it is quite easy to create a modal Dialog in mendix by using dojo's dijit.Dialog. However, in Mendix 5, as Mendix uses Bootstrap, it not easy anymore. -If you still want to use dojo's dijit.Dialog you have to add Dojo' theme manually and change the theme if you want to make it look like mendix' form. -If you want to use bootstrap, you can create form manually by mxui.dom.create... -Use another library (http://xsokev.github.io/Dojo-Bootstrap/) I don't know if there is other way. But I think in mendix, there is already something like mxui/widget/Dialog or mxui/widget/DialogMessage... which is quite similar to dijit.Dialog. Is it correct? If it is correct, It would be nice if Mendix updates the api of those standard components so developers could reuse them and make the look and feel easier.
See https://apidocs.mendix.com/5/client/mx.ui.html, several types of dialogs are available by default in the mx client api