You can add a blip by using the AddBlipForEntity native, or alternatively if the coords of the object are going to be static AddBlipForCoord. You could then style the blip by using more or less any of the natives starting with “SetBlip…”, like SetBlipColour or SetBlipSprite.
If you want the blip to strictly show on the minimap, but not the main map, you could use SetBlipDisplay, and set the displayId to 5 like so: SetBlipDisplay(blip, 5).
In your case this could look something like this:
local blip = AddBlipForEntity(dumpster) -- Adds the blip
SetBlipSprite(blip, 564) -- Sets the icon sprite to "radar_gr_covert_ops"
SetBlipColour(blip, 5) -- Sets the colour to yellow
SetBlipDisplay(blip, 5) -- Only display blip on minimap, not main map
-- Sets blip name
BeginTextCommandSetBlipName("STRING")
AddTextComponentSubstringPlayerName("Emergency Callbox")
EndTextCommandSetBlipName(blip)
Link to blip icons and colours: Blips - Cfx.re Docs
I hope that helps!