The USPTO, like other patent offices, is obligated to treat everyone the same for national applications due to international treaties. But you should check your country’s laws to see if there is a requirement to file locally first or otherwise get permission before filing outside your country.
So, yes, you can file a provisional application (not preliminary) with the USPTO. For a provisional to have any usefulness it needs to be followed up with a non-provisional application within 12 months.
If something is keeping you from filing a non-provisional for 18 months, filing a provisional now won’t help you.
Since a provisional’s substantive content requirements are the same as a non-provisional I can’t image what would keep you from filing a non-provisional for 18 months but contemplate filing a non-provisional now.
We might be able to be more helpful if we understood your 18 month constraint. It might be based on an incorrect assumption.