Assuming you've first done all the one-time setup instruction here:
https://sendoso.zendesk.com/hc/en-us/articles/115002663011-Sendoso-Marketo-Setup-Guide-One-Time-
You are now ready to setup your Marketo Flow steps to trigger the Webhook.
1. Click Marketing Activities and click into the Campaign (or create a new one) where you want to trigger a Sendoso mailer/gift:
2. Within the Campaign, click into the Flow:
3. Add a 'Change Date Value' and select the Attribute as Sendoso Touch ID:
4. Copy/Paste your Unique Touch ID from Sendoso into the New Value field:
*Note- each Sendoso Touch has its' own Touch ID. Each time you want to setup a different Marketo Webhook you'll need to Copy/Paste the specific Touch ID from Sendoso.
5. Add a 'Call Webhook' and select Sendoso:
FYI: Marketo does not support Webhook flow steps in Batch campaigns only Triggered campaigns. If you need to setup a Webhook for a Batch campaign audience then read more here.
That's it!