Hii,
Yes, you can make a text box required only when the "Is custom value" option is selected in Odoo. This is a classic case of conditional field requirement.
Model Code (Python):

from odoo import models, fields, api

class YourModel(models.Model):

    _name = 'your.model'

    attribute_value = fields.Selection([

        ('option1', 'Option 1'),

        ('custom', 'Custom Value')

    ], string="Attribute Value")

    custom_value_text = fields.Char("Custom Value Text")

    is_custom_value = fields.Boolean("Is Custom", compute="_compute_is_custom_value")

    @api.depends('attribute_value')

    def _compute_is_custom_value(self):

        for rec in self:

            rec.is_custom_value = rec.attribute_value == 'custom'

    @api.constrains('attribute_value', 'custom_value_text')

    def _check_custom_value_required(self):

        for rec in self:

            if rec.attribute_value == 'custom' and not rec.custom_value_text:

                raise ValidationError("Custom value must be provided when 'Custom' is selected.")

View Code (XML):
<field name="attribute_value"/>

<field name="custom_value_text"

       invisible =  [('attribute_value', '!=', 'custom')], 'required': [('attribute_value', '=', 'custom')] />
i hope it is use full