How can a flat seller who is living and working overseas, with no Singpass, 2-Factor token or even a Singapore registered mobile number register the Intent to Sell?

All Singapore Citizens, Singapore Permanent Residents, Pass holders (Employment Pass, EntrePass, S-Pass, Dependent Pass, Long Term Visit Pass) and selected Work Permit holders are eligible to register for a Singpass account. They can check their eligibility and register for it via this link: https://www.singpass.gov.sg/singpass/register/instructions
Flat sellers, who are not eligible to register for a Singpass account, can register their Intent to Sell under the ‘I am a non-resident Administrator or Executor’ option. If they do not have a Singapore registered mobile number, they may provide a Singapore registered mobile number of their relative or friend during the registration of their Intent to Sell.