Hi there,
I’ve found two options to do this:
1. [less flexible] Go to Marketing → Lead capture → Forms → Your chosen form
In the tab, you have the ‘submissions’ report and you can save it (right upper corner) to later use it in any dashboard.
2. [more flexible, but works only if you set it up before launching the form]
Create a new date property and name it like ‘Form X submission date’. Next, set up a workflow that will trigger upon Form X submission and will set a time stamp in the ‘Form X submission date’ property. In the settings tab, you can decide whether you want the workflow to only trigger once or always when the contact fills the form. You can then use this property to create a dashboard.
Hope it helps 