I’ve got PRT open on my other PC else I would send a screen shot, but I just did what you are asking in 2 seconds … 3 times / with 3 Indicators under each other.
Click on the symbol bottom left to show the Indicator and backtesting window, select your custom indicator then click Add at the bottom and hey presto all done.
So glad I can help you JuanJ, but somehow I think I may have misunderstood your question?
EDIT / PS
I opened another Platform on this PC, so see attached at red arrowheads.
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