How to enable subscription via SendGrid?

Authorization

To enable SendGrid subscription service you have to follow this instruction.

1. Add your API Key 
2. To create API Key click Settings on the left side menu of your SendGrid account, choose API Keys and click Create API Key button. Type a name for the new key and select specific permissions, that you would like to assign to your new API key. 
3. Click Save button and in the page will appear new API Key which you have to copy. WPCom_Markdown class is not exists
4. Insert your API key in the PopUp plugin settings window and choose a list for subscribe 

Custom fileds

Also you can add Custom fields which will be shown in your pop up. To do it follow the next steps:

1. Go to Marketing -> Custom Fields on the left side menu of dashboard.
2. Click the Create New Field button at the top of the page.
3. Add field name and select field type. Copy the name and click Save. 
4. Go to PopUp plugin setting window and open Subscribe tab of that pop up where you want to add Custom field. Click on Add button 
5. In the Name field insert attribute name you’ve copied in step 3 
6. Add label for this field, click ‘OK’ and save changes.

Now in pop up your visitors will see your new Custom field.