by biboy atienza.
I made it works, by simple using lowercase of the custom field.
https://our-sandbox.mrooms.net/webservice/rest/server.php?wstoken=OUT_TOKEN_HERE&wsfunction=core_user_create_users&moodlewsrestformat=xml&users[0][username]=biboyatienza@gmail.com&users[0][password]=00.00.0000&users[0][firstname]=biboy&users[0][lastname]=atienza&users[0][email]=biboyatienza@gmail.com&users[0][city]=Manila&users[0][country]=Philippines&users[0][customfields][0][type]=customer&users[0][customfields][0][value]=Customer1
Additional info:
If you have more than 1 custom fields, be sure to adjust the array index like below:
&users[0][customfields][0][type]=customer
&users[0][customfields][0][value]=Customer1
&users[0][customfields][1][type]=customeremail
&users[0][customfields][1][value]=customer@email.com
&users[0][customfields][2][type]=customerphone
&users[0][customfields][2][value]=+6325217788
So the link would be :
https://our-sandbox.mrooms.net/webservice/rest/server.php?wstoken=OUT_TOKEN_HERE&wsfunction=core_user_create_users&moodlewsrestformat=xml&users[0][username]=biboyatienza@gmail.com&users[0][password]=00.00.0000&users[0][firstname]=biboy&users[0][lastname]=atienza&users[0][email]=biboyatienza@gmail.com&users[0][city]=Manila&users[0][country]=Philippines&users[0][customfields][0][type]=customer&users[0][customfields][0][value]=Customer1&users[0][customfields][1][type]=customeremail&users[0][customfields][1][value]=customer@email.com&users[0][customfields][2][type]=customerphone&users[0][customfields][2][value]=+6325217788
Hope this will help someone else.
br,
biboyatienza