Here's a basic initiative macro:
[[1d20+@{selected|initiative_bonus}&{tracker}]]
This macro I use as a GM to select monster tokens and add them to the tracker, if you want you players to use it when you create the macro you should have an option under the block for code to make it visible to the players
Here's a bit less basic macro
/desc @{character_name} Is Rolling Initiative: [[1d20+@{initiative_bonus}&{tracker}]]
This macro needs to by adds in every player character under Attributes & Abilities as a new ability
should also check the "show as Token Action" box, This will allow players to select their token and in the top left corner(next to the tools bar) they will a button with the name of the ability in which you entered the above code, clicking the button will print in the chat The following message:
PlayerNameHere Is Rolling Initiative 20