Creating a horizontal form with the Pojo Forms plugin is easy.
The basic rule is that all fields (and button) width should sum up to 100%.
For example, if your form consists of 3 fields and a Submit button, simply set each of the fields' (including the button's) width to 25%.
In order to align the fields with the button, it's better to hide the field's title and insert it as a placeholder instead .
You could configure this in each field's settings.