Hi Ovi,
You’re on the right track to create a separate one for mobile and desktop. However, I wouldn’t worry about the placeholder text on your mobile version. I’d focus on the user experience. Have the input boxes side by side on mobile like that would be very hard for the user to fill out. I’d recommend stacking everything.
That will also fix the placeholder text not showing.
If you’re interest drop down is 3 or less choices, I’d consider doing a radio box (but dropdown works too)
Also, I’m not sure what you’re selling on the site, but I think this pop-up could use a better hook/offer for people to give up their email.