From the Schedule C instructions:

Single-member LLCs. If you are the sole owner of an LLC that is not treated as a separate entity for federal income tax purposes, enter on line D the EIN  that was issued to the LLC (in the LLC's legal name) for a qualified retirement plan, to file employment, excise, alcohol, tobacco, or firearms returns, or as a payer of gambling winnings. If you do not have such an EIN, leave line D blank.

https://www.irs.gov/pub/irs-pdf/i1040sc.pdf

Not every SMLLC is required to have an EIN but if they are then you use the LLC's EIN on line D.