After more research and trial and error, this is what I came up with for it to be on one line. Please make edits to it as you guys wish! I hope this helps others!
form {
display: flex;
align-items: center; /* Vertical alignment */
}
.tnp-subscription {
font-size: 13px;
display: block;
margin: 15px auto;
max-width: 500px;
width: 100%;
}
.tnp-subscription input.tnp-submit {
background-color: #696edb;
color: #fff;
width: auto;
height: auto;
}
.tnp-field-button, .tnp-field-email, .tnp-field-firstname {
max-width: 500px;
width: auto;
height: auto;
margin: 15px auto;
}