I need a php page designed and coded to handle a login and API interaction

General information for the website: We sell financial analysis software and need a license management page for a new product
Num. of web pages/modules: 1
Description of every page/module: I need a page which enables users of our software to change two details of their licensing of the software.
The first detail is the trading account number that the software is licensed to.
The second detail enables them to change the email address their license is linked to.
We need the page to look simple, elegant and classy. Our software is professional, high-end. The page needs to feel secure and slick.
A rough draft of the PHP coding can be provided, which we put together when testing viability of this page.
Description of requirements/features: The page (or pages if necessary) needs to function as follows:
- a user must login (using an email address and a private "key" which they have been provided with during the sales process - that is already tested and working)
- the login is handled by accessing an API - this already works and we have the code for doing that. (I will provide a link to the API documentation, it is fairly straightforward)
- if the login fails we need that handled with a polite message, where to find the private key etc.
- after having logged in the user is presented with all licenses linked to their email address (most of the time it will only be one ... but some users purchase multiple licenses). The API provides the list of licenses (already coded and working)
- each license has only one detail that is relevant ... the trading account number the license is linked to.
- user needs to be able change that account number, preferably in an elegant and foolproof way, such as having a "lightbox" window pop up to edit the number, with the option to cancel if they don't want to complete the change.
- there needs to be a check that the new account number is an integer, no letters or symbols.
- then the new account number is updated using the API
- the page refreshes with the new account number in place.

If the user wishes to change their email address then the process is similar ... all licenses linked to that email address are updated with the API

The page also needs a few other elements elegantly displayed:
- a link to download the install files for the software
- a link to a website where they do an online course in using the software.
- maybe an embedded video which explains software installation (although probably just a link to the video)
Extra services needed: 3rd party integration
Extra notes: The API we need to integrate with is documented here: https://mqllock.com/api2/
I am uploading a few samples of login pages that I like the look of ... I can provide images of financial charts that could be used as a blurred background image perhaps? I need some suggestions and ideas ...