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The addin file format runs as follows:
Field Num Field Description
1 Text:"ApplicationName:Addin"[0D][0A] Fixed text
2 Text:Application name[0D][0A] Variable text
3 Text:Application Description[00] Variable text, null terminated
4 [DD][0A][00] "ADD" in hex, null terminated (previously thought to be a field for the addin size), can be zero too.
5 Icon bitmap (40*32) 32 rows of 5 bytes each bit mapping to a single pixel
6 Start of code Code starting at base address $8000. Ideally starts with:
jp start
This compiles to [C3][03][80], which has the advantage that the start of code can be easily found with a hex editor.

The screenshot below shows these elements marked on a hex editor