How do I specify that I am working as an employee of a non-US company in Singapore (no US social security treaty) and I am not subject to SS /FICA taxes?

If you are a US citizen and are working outside of the US with a net profit from self-employment then you are responsible for paying Self Employment taxes on that income unless you have completed and filed a Certificate of Coverage for a country that has a Totalization Agreement with the US.  

Singapore does not have a Totalization Agreement with the US.

Addendum - 

If you are a US citizen and working outside of the US for a foreign employer, then you enter your income on Form 2555, Foreign Earned Income.  Using TurboTax -

To enter Foreign Earned Income -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Wages and Income (Personal Income using Home and Business)
• Click on I'll choose what I work on (if shown)
• Scroll down to Less Common Income
• On Foreign Earned Income and Exclusion, click on the start or update button

Or enter foreign income in the Search/Find box located in the upper right of the program screen.  Click on Jump to foreign income

For 2017, the maximum exclusion is $102,100 per taxpayer, $103,900 for 2018.