I had a similar requirement where I wanted search by data stored in ref field and was not available in POS Customer search  However I could achieve by modifying the code as below.

In addons/point_of_sale/static/src/js/models.js

i. Search for res.partner (around line 362)

ii. In the fields add ‘ref’ field too.

In addons/point_of_sale/static/src/js/db.js

i. Search for _partner_search_string: function(partner) ( around line 228)

ii. Add if(partner.ref){ str += '|' + partner.ref; } below one of the if condition