Open a Record on the Object you want to add the button to and click Edit Layout.
Objective
How to add a Standard or Custom 'Send for Signature' button on SalesForce Lightning Experience?
Environment
SalesForce
Steps
Following are the steps to add the button once you have created it:
Select 'Mobile & Lightning Actions' from the list.
Drag the button from 'Mobile & Lightning Actions' and add it to 'Salesforce Mobile and Lightning Experience Actions' and save.
The button is available on Lightning now.
Additional information
To create Send for Signature button, refer to Agreement Templates and the Send for Signature Button.
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