How to configure Lookup filter in Salesforce?

Hi,

Follow below steps:

Create a custom field with a lookup filter.

From Setup, click the Object Manager tab.
Click Case, then click Fields & Relationships.
Click Contact Name, then click Edit.
In the Lookup Filter section, click Show Filter Settings.
Click the lookup icon  and choose Contact Name, then Account ID, then click Insert.
Choose equals as the operator.
In the Value/Field menu, select Field.
Click the lookup icon  and choose Case, then Account ID, then click Insert.
Ensure the Required radio button and the Active checkbox are selected.

Click Save.