Updating lookup table value using REST

Hi Everyone,

I am looking for an example on how to add a value to a lookup table using the REST API. Code is being written in C#

For a number of reasons I cannot use the CSOM, namely that a v16 release of the CSOM doesn't exist.

We think we are getting close by posting to the 

http://<sitecollection>/<site>/_api/ProjectServer/LookupTables('tableid')/Entries feed but trying to work out what is expected is a bit of a guessing game.
Can anyone provide some code that does this or point to any examples?
Thanks