How to put main breaker in Square D Hom612L100 breaker panel?
You can do this, but it burns up two panel spaces in the process

While the HOM612L100 is a Main Lug Only panel and thus cannot accept a QOM main frame breaker, it can be fitted with a main breaker.  How?  By backfeeding the panel's busses via a two-pole Homeline branch breaker that is held down with a HOM1RK hold-down kit, as required by NEC 408.36.  As a result, the incoming hot wires go onto the lugs of this held-down breaker, and the main lugs can be left unused, or used as feed-through lugs in case expansion is called for.

Note that this requires the main breaker to be a regular (or triplex/quadruplex) Homeline branch breaker, not a GFCI or AFCI (they cannot be backfed as it would damage their electronics), and also costs two panel spaces in what is already a tiny panel; depending on your application, moving up to say a 12-space panel that accepts a main frame breaker may be a better solution as a result.