I play a hand-built cajun one-row and use a single strap across the right shoulder as well as a thumb strap. The shoulder strap keeps the instrument stable against gravity and against push strokes, the thumb strap allows you to pull while the left hand bass strap keeps the instrument steady. The instrument is held at an angle with the treble side up.
My thumb is fully inserted into the thumb strap, not just to the first knuckle but all the way to the base of the thumb with the pad on the edge of the keyboard. My thumb strap is made of soft leather and is longer than the ones Hohner installs which is both more comfortable and allows my hand to turn and reach keys as needed. Hohner's are just too tight and the leather too stiff. You may need to adjust the attachment of the thumb strap to find the best position for playing. Again, I find Hohner's position to be a bit off. This is the way all the professional one-row players I know do it.
Then you have the mexican style players, they use two shoulder straps and no thumb strap while playing mostly 3-row boxes.