Finally!
Put the generated code on your site.
Generating form...
<form action="https://subscribe.101secure.net/api/subscribe" style="box-sizing: border-box; text-align: center; width: 100%; font-family: sans-serif; background: #ececec; padding: 1.5rem; border: 1px solid #ccc; border-radius: .2rem;">
<input type="hidden" name="GUID" id="GUID" />
<h1 style="line-height: 1; font-size: 1.25rem; margin-top: 0; margin-bottom: 1rem; font-weight: 400;">
Subscribe to our Newsletter
</h1>
<div style="display: flex; flex-direction: column;">
<div style="display: flex; margin-bottom: 1rem;">
<input type="text" id="Firstname" name="Firstname" placeholder="First name" style="width: 100%; box-sizing: border-box; margin-right: .5rem; padding: .25rem .5rem; border-radius: .2rem; border: 1px solid #ccc;" required />
<input type="text" id="Lastname" name="Lastname" placeholder="Last name" style="width: 100%; box-sizing: border-box; margin-left: .5rem; padding: .25rem .5rem; border-radius: .2rem; border: 1px solid #ccc;" required />
</div>
<br />
<input type="email" id="Email" name="Email" placeholder="Email" style="width: 100%; box-sizing: border-box; padding: .25rem .5rem; border-radius: .2rem; border: 1px solid #ccc;" required />
<br />
<button type="submit" style="border: none; border-radius: .2rem; margin-top: 1rem; background-color: #ff6d02; color: #fff; padding: .5rem;">
Subscribe
</button>
</div>
</form>
<form action="https://subscribe.101secure.net/api/subscribe" style="box-sizing: border-box; text-align: center; width: 100%; font-family: sans-serif; background: #ececec; padding: 1.5rem; border: 1px solid #ccc; border-radius: .2rem;">
<input type="hidden" name="GUID" id="GUID" />
<h1 style="line-height: 1; font-size: 1.25rem; margin-top: 0; margin-bottom: 1rem; font-weight: 400;">
Subscribe to our Newsletter
</h1>
<div style="display: flex; flex-direction: column;">
<input type="text" id="Fullname" name="Fullname" placeholder="Full name" style="width: 100%; box-sizing: border-box; padding: .25rem .5rem; border-radius: .2rem; border: 1px solid #ccc;" required />
<br />
<input type="email" id="Email" name="Email" placeholder="Email" style="width: 100%; box-sizing: border-box; padding: .25rem .5rem; border-radius: .2rem; border: 1px solid #ccc;" required />
<br />
<button type="submit" style="border: none; border-radius: .2rem; margin-top: 1rem; background-color: #ff6d02; color: #fff; padding: .5rem;">
Subscribe
</button>
</div>
</form>