I took this example from the reference verbatim into an empty project:
my-mousedown-verb [[src]] [[button]] "keys=[[keys as params]];drag=[[drag as params]]"
client
// "in src" is the same as "in usr.client" here
verb/my_mousedown_verb(object as anything in src, button as text, params as text)
When the macro is used, the following error message appears in the options and messages log:
Sorry, the following is not valid: [0x1000024]
usage: my-mousedown-verb parameter "text" "text"
It looks like it actually needs to be:
client
// "in src" is the same as "in usr.client" here
verb/my_mousedown_verb(object as null|obj|mob|area|turf in usr.client, button as text, params as text)