Custom module form

Hi, I am creating a form in a custom module and would like to have my "who" text field in this custom form use Search Autocompete's autocomplete function for choosing a user. I read the documentation (http://drupal-addict.com/nos-projets/search-autocomplete/advanced-config...) but still cannot get it working. Here is what I have so far:

1. In my custom form, do I need to set the who field's #autocomplete_path key? If yes, what should this be set to?
2. In my custom form, I set the who field '#id' => 'mymodule_who'
3. Using the Search Autocomplete settings page, I created a new form named "Test" and in the Advanced options section, I put "mymodule_who" in the "ID selector for this form" field. If not is not correct, what should I set this to?
4. In this Test form, the "Use an existing view:" is selected and set to "users_autocomplete"

I'm using D7.52, Search Autocomplete 7.x-4.7, PHP 5.6.28

All help/tips are appreciated!

thanks,
Mona