My employer is self-insured. The question is if HSA contributions are considered a "benefit from a self-insured medical plan".
Yes, HSA contributions can be considered a benefit from a self-insured plan (after all, SELF-insured plan can have any benefits they want); but
NO, I doubt that your employer is providing an HDHP plan.
What defines the ability to contribute to an HSA is that you are covered by an HDHP plan (Family or Self-only) and that there is no conflicting coverage (like Medicare, a general purpose health FSA, your spouse's policy that covers you, etc.)
My question is whether or not your employer's self-insured plan actually qualifies as a High Deductible Health Plan (HDHP). There are more requirements for an HDHP than just limits on coverage, so normally, we ask taxpayers to ask their insurance carrier if this is an HDHP.
If your employer tells you that you have an HDHP plan, direct him/her to page 4 of Pub 969, where an HDHP is defined. Note that there are a number of references on the right-hand column that send the insurer to a number of IRS Notices, all of which your employer will have to read.
If your employer doesn't bother to read all that, then you should assume that you policy does not qualify as an HDHP.
If your employer insists that he/she is providing an HDHP, get it in writing. If the IRS discovers that you weren't covered by an HDHP, then all of your contributions may be disallowed.