TID: Mystic BBS 1.12 A48
MSGID: 1:105/420 4a9ada2e
REPLY: 2:335/206 65313515
TZUTC: -0700
 FM> ok, but this is the standard way to launch any program on the floppy
 FM> disk.

To achieve what you showed in the screenshots you don't need anything else. 

 FM> 
 FM> Instead, I wanted to modify the bootblock by inserting a routine inside
 FM> that shows a text like "(c) by Felice Murolo - Press the left mouse
 FM> button" and waits for the left button to be pressed.
 FM> 
 FM> Maybe then enriching this thing with a gradient background color (I've
 FM> already managed to do this, playing with the copper registers).
 FM> 

That's why I asked if you only want to boot from a system or bypass that and
boot as NonDOS disk, which is also possible.

For the second option you need to read:

https://wiki.amigaos.net/wiki/Amiga_Floppy_Boot_Process_and_Physical_Layout

as a starter

-db

... Xerox Alto was the thing. Anything after we use is just a mere copy.

--- Mystic BBS v1.12 A48 (Linux/64)
 * Origin: 2o fOr beeRS bbS>>20ForBeers.com:1337 (1:105/420)
SEEN-BY: 1/123 15/0 18/200 90/1 105/7 10 11 44 81 85 420 500 106/201
SEEN-BY: 123/131 129/305 153/7715 154/10 218/700 226/30 227/114 229/110
SEEN-BY: 229/112 113 206 275 307 317 400 426 428 470 550 664 700 266/512
SEEN-BY: 282/1038 291/111 292/854 305/3 317/3 320/219 322/757 342/200
SEEN-BY: 396/45 460/58 633/280 712/848 5075/35
PATH: 105/420 81 229/426