Hi,

To load a custom field into the POS, you need to define a new function called _loader_params_custom_model(). This function should return a dictionary with a key called search_params. The value of search_params should be a dictionary with two keys. They are domains and fields. The domain should be a list with a tuple specifying filters, and fields should be a list of field names that you want to load into the POS.

For more information refer the blog:

https://www.cybrosys.com/blog/how-to-load-models-and-fields-to-pos-in-odoo-17#:~:text=To%20load%20a%20custom%20field,They%20are%20domains%20and%20fields.

Hope it helps