OK, what you need is just "Set its value with default value, or via rule", as in step 1.
1-a: Double click the field for dynamic url.
1-b: click fx for default value
1-c: set the formula for the dynamic link address, for example, concat("fixed part", field). Click OK
1-d: make sure to check "Refresh value when formula is recalculated"
Or you can set rules for fields that initiate recalculation, if you need more complex condition or calculation for the dynamic address.