TextFrame and In Design

I want to import difference lines of text in a text frame in In Design. Every line can have an other character style (one line is bold, next is light), but i can’t figure out how to do that.

set xtekstkader to 38
set ytekstkader to 133
set btekstkader to 40
set htekstkader to 212
set coordinatenTekstKader to {(ytekstkader - (htekstkader / 2)), (xtekstkader - (btekstkader / 2)), (ytekstkader + (htekstkader / 2)), (xtekstkader + (btekstkader / 2))}
set TekstKaderCredits to make text frame with properties {geometric bounds:coordinatenTekstKader}
tell TekstKaderCredits
-- import text here
end tell

tried things as:

make new text at end with properties {contents:"gsdfgfsdg"}

but that don’t work …