C Spanner there is a locking / adjustable washer at bottom sometimes two, one to adjust and one to lock although many just have one that's how mine work anyway mine are Protechs though :/
I don't think the std bilsteins have threaded collars, if there are groves on the body it may have a locking ring and a sliding collar, or worst case, you need to get a spring spacer to stick in behind the spring to artificially raise the spring platform.
If it doesn't have a threaded collar one can be retrofitted. Alternatively they have a number of grooves that will give some level of ride height adjustment. Just compress spring and move the C-ring under the bottom spring collar up to the next groove.