How to add a custom button on the related list of a record detail page in salesforce

 Problem: As an example, want to show a custom button on the contact related list of the account record.

Step 1: Create a custom button of type 'List Button' on the associated related list i.e. contact.

Go to Setup --> Object Manager --> Contact --> Buttons, Links and Actions section --> click on 'New Button or Link' as shown in the image below

Create a custom Button as shown below by making sure you select display type as 'List Button'.

Step 2: Go to the page layout of the detail page. In this example, associated account page layout.

Click on the settings icon of the associated contact related list and the following screen appears.

Select the respective button in the custom Button section as shown in the image and click on ok.

Save the account page layout and then refresh the account record.

The custom button will now appear as shown in the image below