To add a SignRequest button within Salesforce Lightning in a custom object the following steps are needed:
1) On this page you can find a form where you need to input the name of your object, keep in mind custom objects end with '__c'.
2) Don' t forget to select lightning in the dropdown.
3) Next, copy the complete contents of the snippet.
4) Next, go to Salesforce, Setup > VisualForce Pages > New > And paste the snippet.
5) It's also important to select Available for Lightning Experience, Lightning Communities, and the mobile app.
6) Save, and then go to Setup > ObjectManager > Custom Object > Button Links & Actions and select Detail Page Button, Content Source is VisualForce, and within content you should see the SendSignRequest .
7) Save and go to PageLayout of your Custom Object > Mobile and lightning actions> and your button should show up here now.
8) Maybe you still need to activate custom lightning actions.
9) Now the button should show up at the detail page of your object: