How do I hide the submit button in the modern forms designer?
You can do it with Field Rules.
As long as you have the form assigned to at least task in the Process Diagram, the buttons will show up in the options for the Field Rules.
You can also set the rule to "Hide Always".
This is one of my favorite things about the Layout Designer that the Classic Designer didn't have.
I have it assigned to the message start event task, but I can not see the submit buttons in the designer. So I select a random field and create a new rule, yet all I see is a list of fields to the right of the Show/Hide drop down.
This is a new form with no fields and this is what I see under field rules:
What version are you on?
11.0.2311.50564
I don't seem to have these options, possibly they added it in version 12.
I'm not sure - I thought for sure it was added at least for the last several updates on 11, and that version should indicate you are on the last update of 11.
Forms 11 Update 3 (version 11.0.2212.30907) was when support for showing and hiding action buttons using field rules was added. Very odd that you would not see the behavior if you have a newer version. I have version 12 and also don't see the submit button in the designer but on the left pane when editing the form I do my form name with indented sub options including Field Rules where I can specify to show/hide Action Button: Submit. You shouldn't have to select a random field.
Interestingly enough I was able to replicate your issue if I started by editing a new form without saving my process diagram first. After I saved the process diagram (in my case with a single message start event not even referencing the form I was editing) I was then able to see the Action button: Submit option in the field rules. Perhaps there there needs to be an associated processID in the database to which the field rule can be associated which only occurs after saving the process diagram?
I triple checked that the form is assigned to message start event and going directly to the field rules config (instead of selecting a random field) and I could not find it, but was able to hide it with CSS instead.