Add additional Structured Data (schema) to WP Job Manager when using WP Job Manager Field Editor (baseSalary in this example)

functions.php

<?php

add_filter( 'wpjm_get_job_listing_structured_data', 'smyles_add_field_to_job_structured_data', 10, 2 );

/**
 * Add baseSalary to Structured Data
 *
 * This is just an EXAMPLE of how to set this up, it's up to you to match the correct format required,
 * and to determine what fields to use, and how to set it up based on your site.
 *
 * @param $data
 * @param $post
 *
 * @return mixed
 * @since @@version
 *
 */
function smyles_add_field_to_job_structured_data( $data, $post ){

	if( $post && $post->ID ){
		$salary = get_post_meta( $post->ID, '_job_salary', true );
		
		// Here you can add values that would be considered "not a salary" to skip output for
		$no_salary_values = array( 'Not Disclosed', 'N/A', 'TBD' );
		
		// Don't add anything if empty value, or value equals something above in no salary values
		if( empty( $salary) || in_array( strtolower( $salary ), array_map( 'strtolower' , $no_salary_values ) ) ){
			return $data;
		}
		
		// Determine float value, stripping all non-alphanumeric characters
		$salary_float_val = (float) filter_var( $salary, FILTER_SANITIZE_NUMBER_FLOAT, FILTER_FLAG_ALLOW_FRACTION );
		
		if( ! empty( $salary_float_val ) ){
			// @see https://schema.org/JobPosting
			// Simple value:
			//$data['baseSalary'] = $salary_float_val;
			
			// Or using Google's Structured Data format
			// @see https://developers.google.com/search/docs/data-types/job-posting
			// This is the format Google really wants it in, so you should customize this yourself
			// to match your setup and configuration
			$data['baseSalary'] = array(
				'@type' => 'MonetaryAmount',
				'currency' => 'USD',
				'value' => array(
					'@type' => 'QuantitativeValue',
					'value' => $salary_float_val,
					// HOUR, DAY, WEEK, MONTH, or YEAR
					'unitText' => 'YEAR'
				)
			);
		}
	}

	return $data;
}

Make sure you check https://developers.google.com/search/docs/data-types/job-posting and match the correct format required

https://github.com/Automattic/WP-Job-Manager/blob/master/wp-job-manager-template.php#L363