You would have received an email from Singpass with the subject Registration of Singpass account (Foreign Users), sent during the creation of the Singpass Foreign user Account which contains your Singpass ID.
 

If you have already set up your Singpass Foreign user Account, you can retrieve your Singpass ID online: 

You can retrieve your Singpass ID from this link.

Alternatively:

1. Visit the Singpass portal 

2. Select 'Services' > 'Retrieve Singpass ID' from the top menu bar

3. Scroll to the bottom of the page to access the link for a foreign user

4. Enter your Foreign ID, Country of issuance and email address

5. Your Singpass ID will be sent to your registered email address