Field related with Simple (custom defined list) in a form

Without changing too much or guessing about what you’re trying to do with this form, the below modification would have the expected output:

<style>
	label { margin-bottom: 20px; }
</style>
<label>
	Colección/Serie:
	<select name="coleccion_serie" required>
		<?php
			foreach(
				(array) explode(
					PHP_EOL,
					(string) pods('libro')->fields('coleccion_serie')['pick_custom']
				) as $option
			) {
				list( $value, $label ) = explode( '|', $option );
				printf(
					'<option value="%s">%s</option>',
					esc_attr( $value ),
					esc_html( $label )
				);
			}
		?>
	</select>
</label>

If a specific option should be selected based on the selected value from a specific libro, the libro ID should be passed as the second argument to pods( 'libro', $id ) to get the value with ->field('collection_serie') (singular field, not plural fields for the full definition as used above).

To output selected="selected" for the appropriate option, see selected().

For faster methods of outputting and processing forms using Pods that doesn’t require writing your own form processor, see pods()->form() or add form="1" to a [pods] shortcode with relevant field names separated by commas in attribute fields.

It works perfect, thank you very much.

I usually use shortcodes but there are times when I also need to include the featured_images field or link to taxonomies

Till next time