What you should do is follow this example and use AMPscript in your subject.

"inArguments":[
    { "firstName":"{{Contact.Attribute.__your-de-name__.FirstName}}"},
    { "lastName":"{{Contact.Attribute.__your-de-name__.LastName}}"},
    { "emailAddress": "{{Contact.Default.Email}}"}                          
],

When you fire the event, you can pass data into the DE to be saved. Your custom activity can receive inArgument data to be used within the custom activity. A standard activity like Send Email can pull in DE data via AMPscript.

Also, see this question/answer/comments for more information.

What you should do is follow this example and use AMPscript in your subject.

"inArguments":[
    { "firstName":"{{Contact.Attribute.__your-de-name__.FirstName}}"},
    { "lastName":"{{Contact.Attribute.__your-de-name__.LastName}}"},
    { "emailAddress": "{{Contact.Default.Email}}"}                          
],

When you fire the event, you can pass data into the DE to be saved. Your custom activity can receive inArgument data to be used within the custom activity. A standard activity like Send Email can pull in DE data via AMPscript.

Also, see this question/answer/comments for more information.

What you should do is follow this example and use AMPscript in your subject.

"inArguments":[
    { "firstName":"{{Contact.Attribute.__your-de-name__.FirstName}}"},
    { "lastName":"{{Contact.Attribute.__your-de-name__.LastName}}"},
    { "emailAddress": "{{Contact.Default.Email}}"}                          
],

When you fire the event, you can pass data into the DE to be saved. Your custom activity can receive inArgument data to be used within the custom activity. A standard activity like Send Email can pull in DE data via AMPscript.

What you should do is follow this example and use AMPscript in your subject.

"inArguments":[
    { "firstName":"{{Contact.Attribute.__your-de-name__.FirstName}}"},
    { "lastName":"{{Contact.Attribute.__your-de-name__.LastName}}"},
    { "emailAddress": "{{Contact.Default.Email}}"}                          
],

When you fire the event, you can pass data into the DE to be saved. Your custom activity can receive inArgument data to be used within the custom activity. A standard activity like Send Email can pull in DE data via AMPscript.

Also, see this question/answer/comments for more information.