It's not possible to write a multi-line string into the auto-completion file. The tag file support it, templates support it and also macros.
Disadvantages of tags: No hot keys, no cursor placement after inserting the multi-line string and no auto indentation support. Access to the templates only via the tag list.
Disadvantages of templates: No auto indentation support.
Advantages of templates: Hot keys and cursor placement after insert are possible. A template list also exists.
Advantages of macros: None of the limitations above, if your macro is well written. Macro list also available.
See for example
How to auto complete "if...end if" ?