Similar to most landing pages or emails, you need a CTA that viewers can click to head to the next step. Good CTAs have actionable phrases that let consumers know what they can expect upon clicking. For referral programs, notable CTA phrases include "Refer a Friend", "Get my Referral Code", or "Invite Friends Now".
CTA buttons should stand out from the page so pick a color that stands out from the background. Let's say, your landing page has a white background, your CTA button should be colored in black or red so it attracts people's attention.
The CTA should also be placed in a noticeable location like beside or under the main image. That said, if your referral program landing page has a long FAQ section, don't place the CTA at the bottom of the landing page. Users shouldn't have to scroll far down to find the CTA.
Casper's CTA stands out partly because of the dark blue CTA button contrasting against the white background. On top of this, it is placed at the bottom of the email address text box so users can start sharing afterward.