Generate QR Code tesnet I would like to generate a bitcoin paper testnet wallet.
A paper wallet is a QR code of a private key and a QR code of a bitcoin testnet address, I assume it can be generate juste like this link does : https://api.qrserver.com/v1/create-qr-code/?size=150x150&amp;data=privateKeyOrBtcAddress

But when I'm trying to read it via Testnet3 android app, funds are not recognized. When I read the QR Code from a QR Reader app, I see correctly the content (privateKey for instance).Should I conclude that the application has a problem with this feature or did I miss something ?
Here is an example of taking a 256-bit hex-encoded private key and creating an uncompressed Wallet Input Format (WIF) private key for testnet using the libbitcoin v3.0 bitcoin-explorer (bx) command line interface. bx sub-commands manual pages are located on the right column of its Wiki

% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx base58check-encode -v 239

91avAoeY5k21BDnfaHNLv63frRJhn5RmnEw4XgcsbfpWxPg1A9N

Similarly, here are the results for the associated  compressed public key.  (Note by convention that "01" is tucked on to the private key.)

echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff01" | bx base58check-encode -v 239

cMahgGtgBvsRn7SZFuFCuKGnV8H8yxYLegVcaRsLL2LjD6WFLuAg

More details for using bx for altcoins can be found at Altcoin Version Mapping.  There is a table up top of the wiki that provides insight as to why 239 and 111 are used as versions in bx commands above and below.

Use what ever QR-encoding mechanism you like (e.g., QRencode and encode either 91avAoeY5k21BDnfaHNLv63frRJhn5RmnEw4XgcsbfpWxPg1A9N or cMahgGtgBvsRn7SZFuFCuKGnV8H8yxYLegVcaRsLL2LjD6WFLuAg, but remember the following 256-bit hex encoded private key

0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff 

is one of the worst private keys ever... but makes the point real clear we have a 256-bit hex-encoded key.

Be aware if the funds are deposited to an uncompressed public address and you provide the private WIF key for a compressed address an import or sweep is not likely to work.

The uncompressed public address for the very simple private key can be found by:

% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx ec-to-public -u | bx ec-to-address -v 111

n2nkn4KFNtNbu6Q6GQPZwNZWDiiNsmm9d8

Similarly, the compressed public address can be found by:

% echo "0000111122223333444455556666777788889999aaaabbbbccccddddeeeeffff" | bx ec-to-public | bx ec-to-address -v 111

n47QBape2PcisN2mkHR2YnhqoBr56iPhJh