You can do it by modifying AndroidManifest.xml. After modifying it change it to be a read-only file so it will not be overwritten.
The file is located under the Objects folder.
@Erel
i made a test on b4a 5.02 (1).
i have change on AndroidManifest.xml to landscape and put this read only.
but when compile the b4a generate a new xml file named AndroidManifest-Example.xml
this is portrait, if i change to landscape and save read only again show error on compilation.
B4A version: 5.02 (1)
Parsing code. (0.01s)
Compiling code. Error
O acesso ao caminho 'C:\Users\serig\Desktop\Mega\Mega Play\Objects\AndroidManifest-Example.xml' foi negado.