Table Constraints

I am trying to create a table constraint. 

I have a table named transaction, with a debit account, and a credit account, to move money from one, to the other.

In the accounts I have a users field named "Ninox User" in the debit account, I want the constraint to only show the accounts of the user logged in.

So I am doing:

a.'Debit (From Account)'.'Ninox User'=

but what do I put for it to equal the current user?