It's not clear how are they "blocking" Protonmail (@sender, MX lookups...), but if they are then that's a very lazy (read: incompetent) workaround and I suggest

It's not clear how are they "blocking" Protonmail (@sender, MX lookups...), but if they are then that's a very lazy (read: incompetent) workaround and I suggest you try shopping at a competitor who perhaps use a proper anti-fraud solution. If a corporation of such size deals with abuse by blocking a sender domain name, then their security infrastructure is below poor and it should be an immediate red flag not to trust your sensitive information with them. Or just use a different email address if you really want a laptop, depending on what your priorities are.