On save button of your dataview (form), you can call a microflow. In this microflow add your custom validation logic (required, length etc).
Let say you have to require a name, you can do this in an exclusive split by checking the name attribute != empty and then in its false path, attach a Validation Feedback activity where you can set your custom message for name variable. Whatever you type here as validation message is then displayed in your form.
What do you mean by a custom required field message?
There are two options I can think of:
Neither option is too pleasant, but if you want to be in line with Mendix I would suggest wrapping your inputs in widgets, there are also other input widgets you can analyse to get ideas on how your validation needs to be built.
If my assumption wasn’t correct and you’re just trying to build custom validation, you can always style the display method of your validation with CSS.
Edit: You can assign a class to the inputs and if I remember this correctly the validation div will be situated within the input’s div therefore you can use that for styling.
So you could do something like:
Hope this helps