Login to Learnyst as Admin and go to Website builder. In website builder you can add your custom code by adding a custom code section to your landing page from the side bar. Follow the below steps to do this.
In side bar go to sections and Press Add new section button and a popup bar with section list opens. Click on Custom HTML Code section and the section is added.
In custom code section click on Edit HTML.
Replace the existing custom code with your slide share code and click on Save.
To copy slideshare embed code login into slideshare.com and select the slide you want to share and click on Share. On clicking share button you can see embed code. Copy that code.
Paste the code into custom code section of Learnyst Website Builder. Click on Save and also click on Save on side bar to save these settings and then publish it.