return big data from Automation variable

Hello all!
pls, I have a "little" problem with NAV :-)

I created library in C# and I want use this library as Automation variable in NAV form. Library has function with one parameter and one return value. Both as String (C#).
My problem is with return value because size of text is more as 1024 characters. Using bigtext is also unuseable. ](*,)

My idea is use stream and save return value to tempblob table as blob. But I don't know how make it. On the web I found only example when library save data into local text file and NAV load this data from disk. Not direct as return value.

I tried to use this codes:

InStream.READ(NAV.GetData('Parameter')); => "A variable was excepted... " - not possible compile
InStream.READTEXT(NAV.GetData('Parameter')); => "A variable was excepted... " - not possible compile
BigText.ADDTEXT(NAV.GetData('Parameter')); => compile possible but "The length of the text string exceeds the size of the string buffer"

only for complete information, return string is image, I can show this image in picture box in the Form

Thanks for help!