URL Encoding Issue %3D instead of (=)

Do you mean that you want the submitted form data to look like this:

custom_field_id[]=10=custom_field_value_10=Full-time

In either the URL query string, or in the POST body of an application/x-www-webform-urlencoded request? If so, then no, you cannot do that. That is a violation of both the URL and application/x-www-webform-urlencoded specifications. = is a reserved separator character that cannot appear unencoded in the name and value fields, it must be url-encoded as %3D:

custom_field_id%5B%5D%3D10=custom_field_value_10%3DFull-time

It is the receiver’s responsibility to url-decode the submitted data before then processing it. As such, it would see custom_field_id[]=10 and custom_field_value_10=Full-time values, just as they appeared in the original HTML.