How do I fix it this horizontal forehead dent that could be Botox induced? (Photos)

Thank you for sharing your excellent question and photographs.  This may have occurred due to atrophy of your forehead muscle and can be improved upon with either dermal filler placement or fat grafting.  Hope this helps.

What i see in the photos it's natural shadow between your brows and forehead.  I don't think this is caused be Botox.  I assume you are having your frowner muscles between your eyebrows (glabella) injected?  Hypertrophy of the glabellar muscles could cause the shadow you've noticed.  Theoretically, Botox could help shrink these muscles and provide some improvement.  Otherwise, this is pretty subtle and i would try not to worry about it.  Don't get filler for this.