MB Rank Math helps you to add content of custom fields to Rank Math Content Analysis to have better SEO score.
To let Rank Math get a field's content to analyze, add
'rank_math_analysis' => true to the field:
'name' => 'My custom editor',
'id' => 'custom_content',
'type' => 'wysiwyg',
'rank_math_analysis' => true,
The plugin supports all types of fields, such as text, textarea and wysiwyg.
This is the video tutorial: