If the bespoke formula wants to return an array of values, there are a couple of ways to get the results into multiple cells.
Put the formula into a cell and hit Enter↵. Next, select that cell along with several cells below it. Tap F2 then hit Ctrl+Shift+Enter↵. The successive values should fill the cells selected until an error (no more returns) is reached.
Put that formula into a cell and hit Ctrl+Shift+Enter↵. The formula should be wrapped in braces (e.g. { and }). If the correct relative and absolute cell addresses were used (e.g. $**A$1 or $**A1, etc) then you should be able to fill, copy or drag down the formula into successive rows.
Use an INDEX function to contain the array of returned values from the bespoke formula and peel off successive values using the row_num parameter.
=INDEX(<bespoke formula>, ROW(1:1))
Filled down.
Sooner or later, you will run out of rows to fill. An IFERROR function used as a wrapper can help avoid he display of errors.
If you want to put all of the values into a single cell, then a User defined Function (aka UDF) could concatenate the array into a single string. This last method is generally not recommended as it renders the values useless for anything other than display purposes.
Array formulas need to be finalized with Ctrl+Shift+Enter↵. Once entered into the first cell correctly, they can be filled or copied down or right just like any other formula.
Array formulas chew up calculation cycles logarithmically so it is good practise to narrow the referenced ranges to a minimum.
See Guidelines and examples of array formulas for more information.