I want to be able to write the value "Code" and set the relationship between T1 and T2
That sounds like a description of how you would use a drop down list where you can either select from the value list or type in the code. If this is what you want, just switch your field format in the inspector from "pop up menu" to "drop down list". If your value list is listing values from another table with one value from field 1 and another value field 2 with the show only values from second field option specified, the value from the second field will no longer appear in your field when you exit it. You can correct this issue by adding the field from that related table to your layout and placing it next to the drop down list formatted field.