When editing a vimRC file in VIM, how do you jump to the definition (declaration) of a custom function?

  question, vim

When editing a vimrc file in vim, what shortcut key can be pressed to jump from the place where the custom function (written by vimscript) is called to the place where the custom function is defined (declared)? Do you have any shortcut keys without installing a plug-in like ctags? Note that pressing k (shift+k) can only jump to the description of the system function.

Plug-ins must be installed and are not supported.
In the same file, you can try *, #. There is no guarantee that it can be found, but it can be used.