Hey @ViliBit
In "Custom Form Designer" You can't pass a Variable.
You can follow an Alternate solution:
You can set a default value to the number field and put a condition that checks if the value of this number field is equal to the default value. Then, you can use the "Set Variable" action to pass the variable.

Main Workflow:

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if my suggestion helps you, gives it a Thumbs up 👍 and Marks as a Solution ✔️. Which helps me to Solve more Problems.
Thanks,
Sayan Patra | Consultant RPA Developer