It is also important to note how your specific PSP is operating. My employer's plan would match 50% of your contribution, up to 10%. So if I contributed 10%, then my employer contributed an additional 5%. However, they don't make the matching contributions at the same time as my payroll contributions. They contribute the whole year's employer match as a lump sum at EOY. And if you are no longer working for the company on 12/31, then your matching contributions for that year would instead go into a pot to be distributed among the remaining employees.
Also, regardless of whether your contributions are pre-tax or Roth, employer matches must be a pre-tax contribution since the company gets a tax break on those funds.