Set user role to employer when job submitted using WP Job Manager Field Editor

functions.php

<?php

add_action( 'job_manager_job_submitted', 'smyles_set_user_as_employer_on_submit' );

function smyles_set_user_as_employer_on_submit(){
	if( $user_id = get_current_user_id() ){
		$user = new WP_User( $user_id );

		if( ! in_array( 'employer', (array) $user->roles ) ){
			// You can add the role (keeping subscriber or other roles)
			$user->add_role( 'employer' );
			// Or set to just this role
			// $user->set_role( 'employer' );
		}

	}
}