use variable in user defined formula in project costing

Summary:

Need to use a variable in a user defined formula with the source being the task owning organisation name

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I have created a formula to select a substring from the task owning organisation name in a user defined formula. The user defined formula is substring ('task organization name,5,9) but the formula doesn't return the actual value of the task organization name starting at pos 5. How do I create the formula where the task organization name is used as the variable?

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