Do I literally just send the 1 page resume with salary requirement (if
I decide to include it)? Or do I need to write them a letter or
Send your resume. And send a cover letter stating your salary requirement.
As always, the cover letter should mention the job you are applying for, why you would be a good fit, and should convey your excitement about the possibility of working for the company.
As @Lilienthal correctly points out, when there isn't a specific job posting, your cover letter would instead talk about the kind of profile you have and how your skills and experience would be valuable for the company
Basically, the employer's website is indicating that they want to be sure you include both your resume and salary requirement. They aren't saying that's all you are allowed to send.
And if you decide not to include your salary requirement, they will almost certainly reject your application.