currently, the JDT makes completion proposals only from the context that it finds in the Java sources.
We have template proposals, but those are a separate mechanism that will not blend your proposals into the list computed by the JDT.
Actually, I don't see how the JDT should know in which situations it should consult external lists of your favorite proposals.
So, I don't think your expectation is realistic. I mean, what's so hard about typing "TestKey1"?
Do you have to insert the same string thousands of times? In that case define a constant that will be considered by completion.