Hi,

Please refer to the code below:

Model: Product template line

- Added new field for customer.

from odoo import models, fields

class ProductTemplateLine(models.Model):
    _name = 'product.template.line'
    _description = 'Product Template Line Modul'
    _order = 'x_product_template_id, x_studio_sequence, id'
    _rec_name = 'x_name'

    x_product_template_id = fields.Many2one('product.template', string='Product Template', ondelete='cascade')
    partner_id = fields.Many2one('res.partner', string='Customer')  #ADD THIS LINE
    x_studio_info = fields.Char(string='Info', default='Please enter the PC in Name', required=True)
    x_studio_sequence = fields.Integer(string='Sequence', default=10)
    x_name = fields.Char(string='Description', required=True)
    x_studio_distribution = fields.Selection([
        ('5575', 'Test1'),
        ('5581', 'Test2'),
        ('5559', 'Test3'),
    ], string='Distribution', required=True)
    x_studio_note = fields.Text(string='Note')

Model: purchase order line

from odoo import models, fields, api

class PurchaseOrderLine(models.Model):
    _inherit = 'purchase.order.line'

    x_studio_pc = fields.Many2one('product.template.line', string='PC')

    @api.onchange('product_id', 'order_id.partner_id')
    def _onchange_product_or_partner(self):
        """
        Dynamically filters the available options for the 'x_studio_pc' field 
        based on the selected product and customer.

        Only shows PC entries (product.template.line records) that are linked
        to both the selected product template and the selected customer (partner).
        If either is not selected, the PC dropdown is emptied.
        """
        if self.product_id and self.order_id.partner_id:
            tmpl = self.product_id.product_tmpl_id
            partner = self.order_id.partner_id
            pc_ids = self.env['product.template.line'].search([
                ('x_product_template_id', '=', tmpl.id),
                ('partner_id', '=', partner.id)
            ]).ids
            return {'domain': {'x_studio_pc': [('id', 'in', pc_ids)]}}
        else:
            return {'domain': {'x_studio_pc': []}}

Hope it helps.