Make a SEP contribution to a traditional IRA

I have a traditional IRA with Vanguard that originated as a 401k rollover (so they call it a Rollover IRA). This year I have a lot of self-employment income, and I am qualified to make a SEP IRA contribution of about $20,000 for the previous tax year. Is it possible to make the contribution into my traditional IRA but just "mark" it somehow as a SEP contribution vs. a traditional one? I seem to remember having done this in the past. Vanguard did not respond to my emailed request for information and put me on a long hold and then sent me to someone’s voicemail when I called them.