To redirect customers to a specific step in your Upflowy flow, you will need:
The URL to a published Upflowy flow
A Step ID
To get your URL, copy-paste the URL that's available in the trigger bar:

To get a specific step ID, click into the step that you want your customers to enter on (in this example, I am clicking on the step titled "Industry"):

When you click into the step, look at your URL. It will look like:
https://app.upflowy.com/flows/[UNIQUE FLOW ID/versions/[UNIQUE VERSION ID]/steps/[UNIQUE STEP ID]
...where the unique IDs are a string of numbers and letters. Copy everything after /steps/ in the URL.

Now, take your URL and add onto the end of it:
?u-initial=[UNIQUE STEP ID]
...and congratulations! You now have a link to a specific step in your flow!
