Flexbox isn’t the way to do it.
First, you need to create the fields by answering the question, Starts a New Section?
No. That avoids your problem of the fields being placed within a new parent div.
Then create a field called middle_name
and another called company
Then it’s a bit of trial and error. This works for the theme I was testing with:
#s2member-pro-stripe-registration-form-last-name-div,
#s2member-pro-stripe-registration-form-password-div {
margin-top: 10em;
}
#s2member-pro-stripe-registration-form-custom-reg-field-middle-name-div {
margin-top: -45em !important;
margin-bottom: 16em !important;
}
#s2member-pro-stripe-registration-form-custom-fields-section-title {
visibility: hidden;
}
You will obviously need to play around with the numbers, but you get the idea.